Remove non-standard header values from HttpHeaderValues

Motivation:

x-gzip and x-deflate are not standard header values, and thus should be
removed from HttpHeaderValues, which is meant to provide the standard
values only.

Modifications:

- Remove X_DEFLATE and X_GZIP from HttpHeaderValues
- Move X_DEFLATE and X_GZIP to HttpContentDecompressor and
  DelegatingDecompressorFrameListener
  - We have slight code duplication here, but it does less harm than
    having non-standard constant.

Result:

HttpHeaderValues contains only standard header values.

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

@@ -16,6 +16,7 @@
 package io.netty.handler.codec.http;
 
 import io.netty.channel.embedded.EmbeddedChannel;
+import io.netty.handler.codec.AsciiString;
 import io.netty.handler.codec.compression.ZlibCodecFactory;
 import io.netty.handler.codec.compression.ZlibWrapper;
 
@@ -26,6 +27,15 @@ import io.netty.handler.codec.compression.ZlibWrapper;
  */
 public class HttpContentDecompressor extends HttpContentDecoder {
 
+    /**
+     * {@code "x-deflate"}
+     */
+    private static final AsciiString X_DEFLATE = new AsciiString("x-deflate");
+    /**
+     * {@code "x-gzip"}
+     */
+    private static final AsciiString X_GZIP = new AsciiString("x-gzip");
+
     private final boolean strict;
 
     /**
@@ -48,11 +58,11 @@ public class HttpContentDecompressor extends HttpContentDecoder {
     @Override
     protected EmbeddedChannel newContentDecoder(String contentEncoding) throws Exception {
         if (HttpHeaderValues.GZIP.equalsIgnoreCase(contentEncoding) ||
-            HttpHeaderValues.X_GZIP.equalsIgnoreCase(contentEncoding)) {
+            X_GZIP.equalsIgnoreCase(contentEncoding)) {
             return new EmbeddedChannel(ZlibCodecFactory.newZlibDecoder(ZlibWrapper.GZIP));
         }
         if (HttpHeaderValues.DEFLATE.equalsIgnoreCase(contentEncoding) ||
-            HttpHeaderValues.X_DEFLATE.equalsIgnoreCase(contentEncoding)) {
+            X_DEFLATE.equalsIgnoreCase(contentEncoding)) {
             final ZlibWrapper wrapper = strict ? ZlibWrapper.ZLIB : ZlibWrapper.ZLIB_OR_NONE;
             // To be strict, 'deflate' means ZLIB, but some servers were not implemented correctly.
             return new EmbeddedChannel(ZlibCodecFactory.newZlibDecoder(wrapper));

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

@@ -188,14 +188,6 @@ public final class HttpHeaderValues {
      * {@code "websocket"}
      */
     public static final AsciiString WEBSOCKET = new AsciiString("websocket");
-    /**
-     * {@code "x-deflate"}
-     */
-    public static final AsciiString X_DEFLATE = new AsciiString("x-deflate");
-    /**
-     * {@code "x-gzip"}
-     */
-    public static final AsciiString X_GZIP = new AsciiString("x-gzip");
 
     private HttpHeaderValues() { }
 }

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

@@ -30,6 +30,16 @@ import io.netty.handler.codec.http.HttpHeaderValues;
  * stream.
  */
 public class DelegatingDecompressorFrameListener extends Http2FrameListenerDecorator {
+
+    /**
+     * {@code "x-deflate"}
+     */
+    private static final AsciiString X_DEFLATE = new AsciiString("x-deflate");
+    /**
+     * {@code "x-gzip"}
+     */
+    private static final AsciiString X_GZIP = new AsciiString("x-gzip");
+
     private static final Http2ConnectionAdapter CLEAN_UP_LISTENER = new Http2ConnectionAdapter() {
         @Override
         public void streamRemoved(Http2Stream stream) {
@@ -121,11 +131,11 @@ public class DelegatingDecompressorFrameListener extends Http2FrameListenerDecor
      */
     protected EmbeddedChannel newContentDecompressor(AsciiString contentEncoding) throws Http2Exception {
         if (HttpHeaderValues.GZIP.equalsIgnoreCase(contentEncoding) ||
-            HttpHeaderValues.X_GZIP.equalsIgnoreCase(contentEncoding)) {
+            X_GZIP.equalsIgnoreCase(contentEncoding)) {
             return new EmbeddedChannel(ZlibCodecFactory.newZlibDecoder(ZlibWrapper.GZIP));
         }
         if (HttpHeaderValues.DEFLATE.equalsIgnoreCase(contentEncoding) ||
-            HttpHeaderValues.X_DEFLATE.equalsIgnoreCase(contentEncoding)) {
+            X_DEFLATE.equalsIgnoreCase(contentEncoding)) {
             final ZlibWrapper wrapper = strict ? ZlibWrapper.ZLIB : ZlibWrapper.ZLIB_OR_NONE;
             // To be strict, 'deflate' means ZLIB, but some servers were not implemented correctly.
             return new EmbeddedChannel(ZlibCodecFactory.newZlibDecoder(wrapper));