If HTTP response headers are already sent, don't fiddle with "zlib.output_compression"

Summary:
We write some synthetic HTTP responses inside unit tests. Some responses have an indirect side effect of adjusting "zlib.output_compression", but this adjustment fails if headers have already been output. From a CLI context, headers appear to count as already-output after we write anything to stdout:

```
<?php

echo headers_sent() ? "Y" : "N";
echo "\n";
echo headers_sent() ? "Y" : "N";
echo "\n";
```

This script prints "N", then "Y".

Recently, the default severity of warnings was increased in libphutil; this has been a long-standing warning but now causes test failures.

This behavior is sort of silly but the whole thing is kind of moot anyway. Just skip it if "headers_sent()" is true.

Test Plan: Ran "arc unit --everything", got clean results.

Differential Revision: https://secure.phabricator.com/D21055