subscription push_config does not default to empty hash

[durandom]

I'm getting

[NoMethodError]: undefined method `empty?' for nil:NilClass
/opt/rh/cfme-gemset/gems/fog-google-0.5.2/lib/fog/google/requests/pubsub/create_subscription.rb:29:in `create_subscription'
/opt/rh/cfme-gemset/gems/fog-google-0.5.2/lib/fog/google/models/pubsub/subscription.rb:71:in `save'
/opt/rh/cfme-gemset/gems/fog-core-1.43.0/lib/fog/core/collection.rb:51:in `create'

The calling code is

google.subscriptions.create(:name  => subscription_name, :topic => topic_name)

My guess is either #168 introduced this regression because this line passes push_config and if it's nil it will pass nil.

Adding push_config => {} to the caller fixes the issue.