tests/test_middleware.py: Add test_pop_prefix which exposes the following bug:

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ERROR: test_pop_prefix (tests.test_middleware.WSGIProxyMiddlewareTests)
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Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/WSGIProxy/tests/test_middleware.py",
line 44, in test_pop_prefix
    result = app(environ, start_response)
  File "/Users/marca/dev/git-repos/WSGIProxy/wsgiproxy/middleware.py",
line 115, in __call__
    self._fixup_configured(environ)
  File "/Users/marca/dev/git-repos/WSGIProxy/wsgiproxy/middleware.py",
line 204, in _fixup_configured
    exc = httpexception.HTTPBadRequest(
NameError: global name 'httpexception' is not defined
wsgiproxy/middleware.py: Fix typo `httpexception.HTTPBadRequest` =>
`httpexceptions.HTTPBadRequest`

Fixes GH-7
Add `cover` to .gitignore