Removed a warning that was displayed for HEAD requests.

 > application returned too few bytes (0) for specified Content-Length (85506) via app_iter

I kept getting this warning when doing HEAD requests, but it seems normal to me to get no content in a head request. I made waitress check if the request is a HEAD before issuing the warning.

This patch comes with a test, and a little fix to the tests (first of the two commits).