Add more HttpHeaders values

Motivation:
Some commons values are missing from HttpHeader values constants.

Modifications:
- Add constants for "application/json" Content-Type
- Add constants for "gzip,deflate" Content-Encoding

Result:
More HttpHeader values constants available, both in
`HttpHeaders.Values` and `HttpHeaderValues`.

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

@@ -357,6 +357,10 @@ public abstract class HttpHeaders implements Iterable<Map.Entry<String, String>>
      */
     @Deprecated
     public static final class Values {
+        /**
+         * {@code "application/json"}
+         */
+        public static final String APPLICATION_JSON = "application/json";
         /**
          * {@code "application/x-www-form-urlencoded"}
          */
@@ -406,6 +410,10 @@ public abstract class HttpHeaders implements Iterable<Map.Entry<String, String>>
          * {@code "gzip"}
          */
         public static final String GZIP = "gzip";
+        /**
+         * {@code "gzip,deflate"}
+         */
+        public static final String GZIP_DEFLATE = "gzip,deflate";
         /**
          * {@code "identity"}
          */