hgweb: fix error in 38170eeed18c and introduce test for change

ui: add environ property to access os.environ or wsgirequest.environ The property returns os.environ by default, and is propagated by ui.copy. During hgweb processing, ui.environ is set to the proper WSGI-request environment, as contained in wsgirequest.environ. For CGI, this is the same as os.environ. The property is meant to be read-only, as with os.environ (generally).