Allow to override how headers are encoded

Motivation:

Even if its against the HTTP RFC there are situations where it may be useful to use other chars then US_ASCII in the headers. We should allow to make it possible by allow the user to override the how headers are encoded.

Modifications:

- Add encodeHeaders(...) method and so allow to override it.

Result:

It's now possible to encode headers with other charset then US_ASCII by just extend the encoder and override the encodeHeaders(...) method.

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

@@ -70,7 +70,7 @@ public abstract class HttpObjectEncoder<H extends HttpMessage> extends MessageTo
             buf = ctx.alloc().buffer();
             // Encode the message.
             encodeInitialLine(buf, m);
-            HttpHeaders.encode(m.headers(), buf);
+            encodeHeaders(m.headers(), buf);
             buf.writeBytes(CRLF);
             state = HttpHeaderUtil.isTransferEncodingChunked(m) ? ST_CONTENT_CHUNK : ST_CONTENT_NON_CHUNK;
         }
@@ -133,6 +133,13 @@ public abstract class HttpObjectEncoder<H extends HttpMessage> extends MessageTo
         }
     }
 
+    /**
+     * Encode the {@link HttpHeaders} into a {@link ByteBuf}.
+     */
+    protected void encodeHeaders(HttpHeaders headers, ByteBuf buf) throws Exception {
+        HttpHeaders.encode(headers, buf);
+    }
+
     private void encodeChunkedContent(ChannelHandlerContext ctx, Object msg, long contentLength, List<Object> out) {
         if (contentLength > 0) {
             byte[] length = Long.toHexString(contentLength).getBytes(CharsetUtil.US_ASCII);
@@ -151,9 +158,8 @@ public abstract class HttpObjectEncoder<H extends HttpMessage> extends MessageTo
             } else {
                 ByteBuf buf = ctx.alloc().buffer();
                 buf.writeBytes(ZERO_CRLF);
-
                 try {
-                    HttpHeaders.encode(headers, buf);
+                    encodeHeaders(headers, buf);
                 } catch (Exception ex) {
                     buf.release();
                     PlatformDependent.throwException(ex);