hgweb: validate WSGI environment dict The wsgiref.validate module contains useful functions for validating that various WSGI data structures are proper. This commit adds validation of the environment dict to our built-in HTTP server, which turns an HTTP request into an environment dict. The check discovered that we weren't always setting QUERY_STRING, which would cause the cgi module to fall back to sys.argv. So we change things to always set QUERY_STRING. The check passes on Python 2 and 3. Differential Revision: https://phab.mercurial-scm.org/D2731