middleware,django/init: respect the context passed to #notify

The problem was that when you pass context to notify like this

    bugsnag.notify(Exception("Something broke!"), context="myContext")

Then it's being overwrited by the value for the request_config.

Probably, grouping_hash has the similar issue.
@kyrylo Unless I'm missing something, `notification.context` is guaranteed to be overridden by either line 27, 31, or 33. So we should probably not set it here, and remove the `context` local variable.