R1:067b04aaf148

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…