In order for web.Wrap to be able to detect if a response has been written
we need to wrap any non-context.ResponseWriters as a such. Otherwise
responses will be incorrectly detected as non-written to and handlers can
double run.

In the case of GZip this handler will change the response to a non-context.RW
and this failure to correctly detect response writing causes fallthrough and
a NPE.

Fix go-gitea#19839

Signed-off-by: Andrew Thornton <art27@cantab.net>