This exception caused serious trouble, e.g.:
- Clients got an invalid/incomplete packet, which breaks them.
- Openfire logged related exceptions like org.jivesoftware.openfire.http.HttpConnectionClosedException: The http connection is no longer available to deliver content

It was caused by calling complete() although the previous write() has not completed (due to its async nature).

I also restructured that the Content-Length header is only set once.

Fixes OF-989.
See also https://community.igniterealtime.org/thread/57622