httpserver: If no X-Scheme header, use the normal request value.

Previously, if xheaders is True and there are no X headers passed (e.g. when
developing locally), scheme was always "http". This makes in "http" or
"https", based on what was actually used for the request.

Add tests for the X-Scheme and X-Forwarded-Proto headers.
Merge pull request #695 from evanj/master

httpserver: If no X-Scheme header, use the normal request value.