add flushBuffer

Author: chengyouling

common/common-rest/src/main/java/org/apache/servicecomb/common/rest/filter/inner/ServerRestArgsFilter.java

@@ -19,6 +19,7 @@
 
 import static org.apache.servicecomb.common.rest.filter.inner.RestServerCodecFilter.isDownloadFileResponseType;
 
+import java.io.IOException;
 import java.util.Map;
 import java.util.concurrent.CompletableFuture;
 
@@ -167,6 +168,11 @@ private static CompletableFuture<Response> writeResponse(
         if (e != null) {
           result.completeExceptionally(e);
         }
+        try {
+          responseEx.flushBuffer();
+        } catch (IOException ex) {
+          LOGGER.warn("Failed to flush buffer for Server Send Events", ex);
+        }
       });
       result.complete(response);
       return result;