Use context passed in as parameter instead of +MR_contextForCurrentThrea...

...d

Previously, this method used the context supplied as a parameter
to create the fetch request, but then used `[NSManagedObjectContext
MR_contextForCurrentThread]` to create the
`NSFetchedResultsController`.
It will now correctly use the provided context for both the fetch
request and the `NSFetchedResultsController`.