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.
I store the context passed to `#notify` on line 13. Usually it's "nil", but if the user wants to modify it, then it's a string. Line 27 restores the context after modifying it on line 14, but only if the user provided their own context. If not, then we let the notifier to decide how it should look like. I just came to conclusion that line 14 is redundant. I agree that it can be written more understandable.