Fix issuse #1721 Accept-Encoding header is added twice (#2139)

* Fix issuse #1721 Accept-Encoding header is added twice

* format core code

core/src/main/java/feign/Client.java

@@ -14,6 +14,7 @@
 package feign;
 
 import static feign.Util.CONTENT_ENCODING;
+import static feign.Util.ACCEPT_ENCODING;
 import static feign.Util.CONTENT_LENGTH;
 import static feign.Util.ENCODING_DEFLATE;
 import static feign.Util.ENCODING_GZIP;
@@ -186,6 +187,11 @@ public interface Client {
               contentLength = Integer.valueOf(value);
               connection.addRequestProperty(field, value);
             }
+          }
+          // Avoid add "Accept-encoding" twice or more when "compression" option is enabled
+          if (field.equals(ACCEPT_ENCODING)) {
+            connection.addRequestProperty(field, String.join(", ", request.headers().get(field)));
+            break;
           } else {
             connection.addRequestProperty(field, value);
           }

core/src/main/java/feign/Util.java

@@ -64,6 +64,10 @@ public class Util {
    * The HTTP Content-Encoding header field name.
    */
   public static final String CONTENT_ENCODING = "Content-Encoding";
+  /**
+   * The HTTP Accept-Encoding header field name.
+   */
+  public static final String ACCEPT_ENCODING = "Accept-Encoding";
   /**
    * The HTTP Retry-After header field name.
    */