[UNDERTOW-2243] At ServletOutputStreamImpl.updateWritten, flush the c…

…hannel instead of closing it if full content length has been written

Signed-off-by: Flavia Rainone <frainone@redhat.com>

servlet/src/main/java/io/undertow/servlet/spec/ServletOutputStreamImpl.java

@@ -370,7 +370,7 @@ void updateWritten(final long len) throws IOException {
         this.written += len;
         long contentLength = servletRequestContext.getOriginalResponse().getContentLength();
         if (contentLength != -1 && this.written >= contentLength) {
-            close();
+            flushInternal();
         }
     }