Add missing file header

Author: slandelle

client/src/main/java/org/asynchttpclient/util/HttpConstants.java

@@ -3,6 +3,19 @@
 import io.netty.handler.codec.http.HttpMethod;
 import io.netty.handler.codec.http.HttpResponseStatus;
 
+/*
+ * Copyright (c) 2015 AsyncHttpClient Project. All rights reserved.
+ *
+ * This program is licensed to you under the Apache License Version 2.0,
+ * and you may not use this file except in compliance with the Apache License Version 2.0.
+ * You may obtain a copy of the Apache License Version 2.0 at
+ *     http://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the Apache License Version 2.0 is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the Apache License Version 2.0 for the specific language governing permissions and limitations there under.
+ */
 public final class HttpConstants {
 
     public static final class Methods {