Introduce MessageAggregator and DecoderResultProvider

Motivation:

We have different message aggregator implementations for different
protocols, but they are very similar with each other.  They all stems
from HttpObjectAggregator.  If we provide an abstract class that provide
generic message aggregation functionality, we will remove their code
duplication.

Modifications:

- Add MessageAggregator which provides generic message aggregation
- Reimplement all existing aggregators using MessageAggregator
- Add DecoderResultProvider interface and extend it wherever possible so
  that MessageAggregator respects the state of the decoded message

Result:

Less code duplication

codec-http/src/main/java/io/netty/handler/codec/http/ComposedLastHttpContent.java

@@ -70,7 +70,7 @@ public ByteBuf content() {
     }
 
     @Override
-    public DecoderResult getDecoderResult() {
+    public DecoderResult decoderResult() {
         return result;
     }
 

codec-http/src/main/java/io/netty/handler/codec/http/DefaultHttpContent.java

@@ -92,6 +92,6 @@ public boolean release(int decrement) {
     @Override
     public String toString() {
         return StringUtil.simpleClassName(this) +
-               "(data: " + content() + ", decoderResult: " + getDecoderResult() + ')';
+               "(data: " + content() + ", decoderResult: " + decoderResult() + ')';
     }
 }

codec-http/src/main/java/io/netty/handler/codec/http/DefaultHttpObject.java

@@ -26,7 +26,7 @@ protected DefaultHttpObject() {
     }
 
     @Override
-    public DecoderResult getDecoderResult() {
+    public DecoderResult decoderResult() {
         return decoderResult;
     }
 

codec-http/src/main/java/io/netty/handler/codec/http/DefaultHttpRequest.java

@@ -95,7 +95,7 @@ public String toString() {
         StringBuilder buf = new StringBuilder();
         buf.append(StringUtil.simpleClassName(this));
         buf.append("(decodeResult: ");
-        buf.append(getDecoderResult());
+        buf.append(decoderResult());
         buf.append(')');
         buf.append(StringUtil.NEWLINE);
         buf.append(getMethod());

codec-http/src/main/java/io/netty/handler/codec/http/DefaultHttpResponse.java

@@ -74,12 +74,12 @@ public String toString() {
         StringBuilder buf = new StringBuilder();
         buf.append(StringUtil.simpleClassName(this));
         buf.append("(decodeResult: ");
-        buf.append(getDecoderResult());
+        buf.append(decoderResult());
         buf.append(')');
         buf.append(StringUtil.NEWLINE);
         buf.append(getProtocolVersion().text());
         buf.append(' ');
-        buf.append(getStatus().toString());
+        buf.append(getStatus());
         buf.append(StringUtil.NEWLINE);
         appendHeaders(buf);
 

codec-http/src/main/java/io/netty/handler/codec/http/HttpHeaders.java

@@ -36,7 +36,7 @@
  */
 public abstract class HttpHeaders implements Iterable<Map.Entry<String, String>> {
 
-    private static final byte[] HEADER_SEPERATOR = { HttpConstants.COLON, HttpConstants.SP };
+    private static final byte[] HEADER_SEPERATOR = { COLON, SP };
     private static final byte[] CRLF = { CR, LF };
 
     public static final HttpHeaders EMPTY_HEADERS = new HttpHeaders() {
@@ -722,7 +722,7 @@ public static int getIntHeader(HttpMessage message, CharSequence name, int defau
 
         try {
             return Integer.parseInt(value);
-        } catch (NumberFormatException e) {
+        } catch (NumberFormatException ignored) {
             return defaultValue;
         }
     }
@@ -783,7 +783,7 @@ public static Date getDateHeader(HttpMessage message, CharSequence name, Date de
 
         try {
             return HttpHeaderDateFormat.get().parse(value);
-        } catch (ParseException e) {
+        } catch (ParseException ignored) {
             return defaultValue;
         }
     }
@@ -865,7 +865,7 @@ public static long getContentLength(HttpMessage message, long defaultValue) {
         if (contentLength != null) {
             try {
                 return Long.parseLong(contentLength);
-            } catch (NumberFormatException e) {
+            } catch (NumberFormatException ignored) {
                 return defaultValue;
             }
         }
@@ -1408,7 +1408,12 @@ public HttpHeaders set(HttpHeaders headers) {
         if (headers == null) {
             throw new NullPointerException("headers");
         }
+
         clear();
+        if (headers.isEmpty()) {
+            return this;
+        }
+
         for (Map.Entry<String, String> e: headers) {
             add(e.getKey(), e.getValue());
         }

codec-http/src/main/java/io/netty/handler/codec/http/HttpObject.java

@@ -15,17 +15,6 @@
  */
 package io.netty.handler.codec.http;
 
-import io.netty.handler.codec.DecoderResult;
+import io.netty.handler.codec.DecoderResultProvider;
 
-public interface HttpObject {
-    /**
-     * Returns the result of decoding this message.
-     */
-    DecoderResult getDecoderResult();
-
-    /**
-     * Updates the result of decoding this message. This method is supposed to be invoked by {@link HttpObjectDecoder}.
-     * Do not call this method unless you know what you are doing.
-     */
-    void setDecoderResult(DecoderResult result);
-}
+public interface HttpObject extends DecoderResultProvider { }