Handle strings in WSGI environment correctly

According to PEP-3333, "CGI-defined variables must be native strings, if they are present at all". WebTest right now doesn't honor that, since it compares PATH_INFO to a unicode '/' -- which breaks with UnicodeDecodeError when PATH_INFO contains non-ASCII characters. (An example test that suffers from this is `test_highchars_in_filename` from Pyramid; note that this test is run only if your locale is something other than C.)