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

======================================================================
ERROR: test_trust_ips (tests.test_middleware.WSGIProxyMiddlewareTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/WSGIProxy/tests/test_middleware.py",
line 52, in test_trust_ips
    result = app(environ, start_response)
  File "/Users/marca/dev/git-repos/WSGIProxy/wsgiproxy/middleware.py",
line 113, in __call__
    self._fixup_environ(environ, start_response)
  File "/Users/marca/dev/git-repos/WSGIProxy/wsgiproxy/middleware.py",
line 133, in _fixup_environ
    if ip in trust_ips:
NameError: global name 'trust_ips' is not defined
wsgiproxy/middleware.py: Correct typo `trust_ips` => `self.trust_ips`

Fixes GH-8