Update DevServer to handle empty POST requests (#1068)

Set Content-Length to 0 and close output stream after writing.

So that mvn appengine:stop works again (same in Gradle)

Author: ludoch

appengine-plugins-core/src/main/java/com/google/cloud/tools/appengine/operations/DevServer.java

@@ -205,7 +205,8 @@ public void stop(StopConfiguration configuration) throws AppEngineException {
       connection.setDoOutput(true);
       connection.setDoInput(true);
       connection.setRequestMethod("POST");
-      connection.getOutputStream().write('\n');
+      connection.setRequestProperty("Content-Length", "0");
+      connection.getOutputStream().close();
       connection.disconnect();
       int responseCode = connection.getResponseCode();
       if (responseCode < 200 || responseCode > 299) {