downloadHandler is null on DELETE methods.

Found this bug when I was trying to delete a friend.  It deleted just fine but then threw a null exception and stopped all execution.

Turns out www.downloadHandler wasn't available when the method is DELETE .  I'm not sure if this is intentional, but this should fix it.

Assets/Nakama/UnityWebRequestAdapter.cs

@@ -127,8 +127,8 @@ private static IEnumerator SendRequest(UnityWebRequest www, Action<string> callb
             }
             else
             {
-                callback?.Invoke(www.downloadHandler.text);
+                callback?.Invoke(www.downloadHandler?.text);
             }
         }
     }
-}
+}