* ensure compressing stream wrapper is closed, hence compressors are
flushed
* handle streaming output (also move header manipulation up to be
visible in streaming mode)
* ensure compression stream is wrapped only once (vs. each time a writer
to the output message calls `getBody`)

Also...

* extracted the plugin to it's own artifact to avoid module cycles
* extended the tests to actually verify (de-)compression for a set of
request factories