Fixed case of writing a zero-length buffer that is not the last content:

we should return FLUSH instead of DONE, waiting for the last content flag.

jetty-http/src/main/java/org/eclipse/jetty/http/HttpGenerator.java

@@ -255,7 +255,7 @@ public class HttpGenerator
                     return len>0?Result.FLUSH:Result.CONTINUE;
                 }
 
-                return len>0?Result.FLUSH:Result.DONE;
+                return Result.FLUSH;
             }
 
             case COMPLETING: