Merge pull request #1178 from marciomeschini/marco/fix_accept_header

add webp accepted content type only if WebP enabled

@@ -67,7 +67,11 @@ static NSString *const kCompletedCallbackKey = @"completed";
         _downloadQueue = [NSOperationQueue new];
         _downloadQueue.maxConcurrentOperationCount = 6;
         _URLCallbacks = [NSMutableDictionary new];
-        _HTTPHeaders = [NSMutableDictionary dictionaryWithObject:@"image/webp,image/*;q=0.8" forKey:@"Accept"];
+#ifdef SD_WEBP
+        _HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy];
+#else
+        _HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy];
+#endif
         _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
         _downloadTimeout = 15.0;
     }