ByteString arrayOffset method

Motivation:
The ByteString class currently assumes the underlying array will be a complete representation of data. This is limiting as it does not allow a subsection of another array to be used. The forces copy operations to take place to compensate for the lack of API support.

Modifications:
- add arrayOffset method to ByteString
- modify all ByteString and AsciiString methods that loop over or index into the underlying array to use this offset
- update all code that uses ByteString.array to ensure it accounts for the offset
- add unit tests to test the implementation respects the offset

Result:
ByteString and AsciiString can represent a sub region of a byte[].

buffer/src/main/java/io/netty/buffer/ByteBufUtil.java

@@ -610,7 +610,7 @@ public static void copy(ByteString src, int srcIdx, ByteBuf dst, int dstIdx, int
                             + length + ") <= srcLen(" + thisLen + ')');
         }
 
-        checkNotNull(dst, "dst").setBytes(dstIdx, src.array(), srcIdx, length);
+        checkNotNull(dst, "dst").setBytes(dstIdx, src.array(), srcIdx + src.arrayOffset(), length);
     }
 
     /**
@@ -628,7 +628,7 @@ public static void copy(ByteString src, int srcIdx, ByteBuf dst, int length) {
                             + length + ") <= srcLen(" + thisLen + ')');
         }
 
-        checkNotNull(dst, "dst").writeBytes(src.array(), srcIdx, length);
+        checkNotNull(dst, "dst").writeBytes(src.array(), srcIdx + src.arrayOffset(), length);
     }
 
     static final class ThreadLocalUnsafeDirectByteBuf extends UnpooledUnsafeDirectByteBuf {

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2HeadersEncoder.java

@@ -109,7 +109,10 @@ public Configuration configuration() {
     }
 
     private void encodeHeader(ByteString key, ByteString value, OutputStream stream) throws IOException {
-        encoder.encodeHeader(stream, key.array(), value.array(), sensitivityDetector.isSensitive(key, value));
+        encoder.encodeHeader(stream,
+                key.isEntireArrayUsed() ? key.array() : new ByteString(key, true).array(),
+                value.isEntireArrayUsed() ? value.array() : new ByteString(value, true).array(),
+                sensitivityDetector.isSensitive(key, value));
     }
 
     /**

codec-http2/src/main/java/io/netty/handler/codec/http2/HttpUtil.java

@@ -377,7 +377,7 @@ public boolean visit(Entry<ByteString, ByteString> entry) throws Http2Exception
                     throw streamError(streamId, PROTOCOL_ERROR,
                             "Invalid HTTP/2 header '%s' encountered in translation to HTTP/1.x", translatedName);
                 } else {
-                    output.add(new AsciiString(translatedName.array(), false), new AsciiString(value.array(), false));
+                    output.add(new AsciiString(translatedName, false), new AsciiString(value, false));
                 }
             }
             return true;