tests/test_middleware.py: Make start_response take optional exc_info
argument. Improve and uncomment test_exception_HTTPBadRequest.
Pass start_response into _fixup_environ. Prevents an error:

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ERROR: test_exception_HTTPBadRequest (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 26, in test_exception_HTTPBadRequest
    app(environ, start_response)
  File "/Users/marca/dev/git-repos/WSGIProxy/wsgiproxy/middleware.py", line 113, in __call__
    self._fixup_environ(environ)
  File "/Users/marca/dev/git-repos/WSGIProxy/wsgiproxy/middleware.py", line 162, in _fixup_environ
    return exc(environ, start_response)
NameError: global name 'start_response' is not defined
tests/test_middleware.py: Check the results returned by calls to WSGI
apps.