Failed charge webhooks

[meshy]

When dealing with the webhook event after using the stripe test card 4000 0000 0000 0341, I was getting the following error:

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Internal Server Error: /payments/webhook/
Traceback (most recent call last):
  File "/.../django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/.../channels/handler.py", line 228, in process_exception_by_middleware
    return super(AsgiHandler, self).process_exception_by_middleware(exception, request)
  File "/.../django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib64/python3.4/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/.../django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/.../django/utils/decorators.py", line 67, in _wrapper
    return bound_func(*args, **kwargs)
  File "/.../django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/.../django/utils/decorators.py", line 63, in bound_func
    return func.__get__(self, type(self))(*args2, **kwargs2)
  File "/.../pinax/stripe/views.py", line 185, in dispatch
    return super(Webhook, self).dispatch(*args, **kwargs)
  File "/.../django/views/generic/base.py", line 88, in dispatch
    return handler(request, *args, **kwargs)
  File "/.../pinax/stripe/views.py", line 200, in post
    message=data
  File "/.../pinax/stripe/actions/events.py", line 30, in add_event
    webhook.process()
  File "/.../pinax/stripe/webhooks.py", line 91, in process
    self.process_webhook()
  File "/.../pinax/stripe/webhooks.py", line 171, in process_webhook
    stripe.Charge.retrieve(self.event.message["data"]["object"]["id"])
  File "/.../pinax/stripe/actions/charges.py", line 99, in sync_charge_from_stripe_data
    customer = models.Customer.objects.get(stripe_id=data["customer"])
  File "/.../django/db/models/manager.py", line 122, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/.../django/db/models/query.py", line 387, in get
    self.model._meta.object_name
pinax.stripe.models.DoesNotExist: Customer matching query does not exist.

This is because the json data returned by the webook contains "customer": null,.

This PR introduces a check to make sure the customer ID is in the JSON data before attempting to get a Customer based on it.

Currently this fails because Charge.customer is not a nullable field. That appears to have already been addressed in #307.

[meshy]

I've just noticed that #307 takes a different approach to this.

The code is shorter, but does a lookup that might not be required.

[meshy]

@paltman thank you for the flurry of merges! As this PR is now reduced to a single test case that (due to your merges) now passes, I would understand if you would like to close this without merging -- I'll leave it open for the moment, and in your hands.

[paltman]

More tests the better!

Thanks!