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).