Copy acceptsGzipEncoding method from gwtjsonrpc

This is a tiny utility that we don't need to bring in a whole external
dependency for.

Change-Id: I3d5d4e9f8d9272861c890ad59fca1662b05d608c

java/com/google/gerrit/util/http/RequestUtil.java

@@ -89,5 +89,10 @@ public class RequestUtil {
     return result.toString();
   }
 
+  public static boolean acceptsGzipEncoding(HttpServletRequest request) {
+    String accepts = request.getHeader("Accept-Encoding");
+    return accepts != null && accepts.indexOf("gzip") != -1;
+  }
+
   private RequestUtil() {}
 }