GAE Backend loading issues

[martynsmith]

When I try to import pusher on my GAE project, I'm getting a stack trace something like this:

  ...
  File "my-project/mymodule.py", line 10, in <module>
    from pusher import Pusher
  File "lib/pusher/__init__.py", line 3, in <module>
    from .pusher import Pusher
  File "lib/pusher/pusher.py", line 7, in <module>
    from pusher.requests import RequestsBackend
  File "lib/pusher/requests.py", line 12, in <module>
    import urllib3.contrib.pyopenssl
  File "lib/urllib3/contrib/pyopenssl.py", line 48, in <module>
    from ndg.httpsclient.ssl_peer_verification import SUBJ_ALT_NAME_SUPPORT
ImportError: No module named ndg.httpsclient.ssl_peer_verification

This seems to be some issue around GAE not including the necessary modules that RequestsBackend requires.

This patch changes the default backend behaviour such that it only imports the default if one wasn't specified in the constructor (avoiding this issue for GAE users, and potentially others that don't have requests installed).

[zimbatm]

Thanks, makes sense