[#1309] Make sure ReplayDecoder respect isSingleDecode()

* This could cause for example corrupt WebSocketFrame's if they was written from the server
  to the client directly after it send the handshake response.

codec/src/main/java/io/netty/handler/codec/ReplayingDecoder.java

@@ -427,6 +427,9 @@ public abstract class ReplayingDecoder<S> extends ByteToMessageDecoder {
                            "if it returned a decoded message (caused by: " +
                            getClass() + ')');
                 }
+                if (isSingleDecode()) {
+                    break;
+                }
             }
         } catch (CodecException e) {
             throw e;

codec/src/test/java/io/netty/handler/codec/ReplayingDecoderTest.java

@@ -85,4 +85,20 @@ public class ReplayingDecoderTest {
             ctx.pipeline().replace(this, "less-bloated", new LineDecoder());
         }
     }
+
+    @Test
+    public void testSingleDecode() throws Exception {
+        LineDecoder decoder = new LineDecoder();
+        decoder.setSingleDecode(true);
+        EmbeddedByteChannel ch = new EmbeddedByteChannel(decoder);
+
+        // "C\n" should be appended to "AB" so that LineDecoder decodes it correctly.
+        ch.writeInbound(Unpooled.wrappedBuffer(new byte[]{'C', '\n' , 'B', '\n'}));
+        assertEquals(Unpooled.wrappedBuffer(new byte[] {'C' }), ch.readInbound());
+        assertNull("Must be null as it must only decode one frame", ch.readInbound());
+
+        ch.finish();
+        assertEquals(Unpooled.wrappedBuffer(new byte[] {'B' }), ch.readInbound());
+        assertNull(ch.readInbound());
+    }
 }