NativeHTTPRequest: Fix buffer management for neko

src/lime/_internal/backend/native/NativeHTTPRequest.hx

@@ -394,7 +394,8 @@ class NativeHTTPRequest
 
 	private function curl_onWrite(curl:CURL, output:Bytes):Int
 	{	
-		buffer.add(output);
+		buffer.addBytes(output, 0, output.length)
+					
 		return output.length;
 	}