fix unit test failed. (sofastack#681)

* fix unit test failed.

* Avoid null key transfer to headers

extension-impl/remoting-bolt/src/main/java/com/alipay/sofa/rpc/codec/bolt/SimpleMapSerializer.java

@@ -55,8 +55,13 @@ public byte[] encode(Map<String, String> map) throws SerializationException {
             for (Map.Entry<String, String> entry : map.entrySet()) {
                 String key = entry.getKey();
                 String value = entry.getValue();
-                writeSupportEmpty(key, out);
-                writeSupportEmpty(value, out);
+                /**
+                 * 排除不写null作为key
+                 */
+                if(key != null){
+                    writeSupportEmpty(key, out);
+                    writeSupportEmpty(value, out);
+                }
             }
             return out.toByteArray();
         } catch (IOException ex) {

extension-impl/remoting-bolt/src/test/java/com/alipay/sofa/rpc/codec/bolt/SimpleMapSerializerTest.java

@@ -44,15 +44,15 @@ public void encode() throws Exception {
         map.put("a", "");
         map.put("b", null);
         bs = simpleMapSerializer.encode(map);
-        Assert.assertEquals(10, bs.length);
+        Assert.assertEquals(37, bs.length);
 
         Map<String, String> map1 = simpleMapSerializer.decode(bs);
         Assert.assertNotNull(map1);
-        Assert.assertEquals(1, map1.size());
+        Assert.assertEquals(4, map1.size());
         Assert.assertEquals("2", map1.get("1"));
-        Assert.assertEquals(null, map1.get(""));
-        Assert.assertEquals(null, map1.get("a"));
-        Assert.assertEquals(null, map1.get("b"));
+        Assert.assertEquals("x", map1.get(""));
+        Assert.assertEquals("", map1.get("a"));
+        Assert.assertEquals("", map1.get("b"));
 
         map1 = simpleMapSerializer.decode(null);
         Assert.assertNotNull(map1);