The icon does not match the others because the NuvolaX icon theme seems
to have vanished from the Internet. Ideally all those admin icons could
use a replacement. But that should be a different PR I guess.

The pragma header is only defined for requests not for responses. The
Cache-Control header should be used in responses.

- put test comments in more appropriate spot
- move appending replacement line alongside its search/delete code

The changes should also:
- fix unlikely edge case when replacement line is the same as the
  old line (would have resulted in timeout)
- reduce memory footprint
- avoid applying string search beyond maxlines replacement limit