PATCH call not proxied correctly to active node (#6475) (#6522)

Summary: added a workaround for http methods unsupported by java

JIRA issues: MARATHON-8095

(cherry picked from commit deac8cd)

src/main/scala/mesosphere/marathon/api/forwarder/JavaUrlConnectionRequestForwarder.scala

@@ -105,7 +105,7 @@ class JavaUrlConnectionRequestForwarder(
   }
 
   private def copyRequestToConnection(leaderConnection: HttpURLConnection, request: HttpServletRequest): Try[Done] = Try {
-    leaderConnection.setRequestMethod(request.getMethod)
+    HttpUrlConnectionWorkaround.setMethod(leaderConnection, request.getMethod)
     copyRequestHeadersToConnection(leaderConnection, request)
     copyRequestBodyToConnection(leaderConnection, request)
     Done
@@ -214,3 +214,18 @@ object JavaUrlConnectionRequestForwarder extends StrictLogging {
     }
   }
 }
+
+/**
+  * Workaround to overcome Java restrictions; see https://bugs.openjdk.java.net/browse/JDK-7016595
+  */
+private[forwarder] object HttpUrlConnectionWorkaround {
+  private val methodsField = classOf[HttpURLConnection].getDeclaredField("method")
+  methodsField.setAccessible(true)
+
+  def setMethod(connection: HttpURLConnection, method: String): Unit = {
+    if (method == "PATCH")
+      methodsField.set(connection, method)
+    else
+      connection.setRequestMethod(method)
+  }
+}