Properly clear user/tags/extra context after request.

As brought up at https://github.com/getsentry/raven-ruby/commit/0bf46f9e957b179a5d0ce1295840eaff89d8146a#commitcomment-2775575.

Me 8 days ago:

> Either I'm crazy or `@context ||= Context.new` should be `Context.current`. Which is it?  :smile: 

Today:

> Worse even, I think we shouldn't be calling `Context.new` or `Raven.context` anywhere. We should only be accessing the context through `Context.current` (where `current` and `clear!` should become class methods), so clearing the context in `Raven:Rack` using `Context.clear!` will actually result in a new context being used when `Raven.context` or `Context.current` is called a subsequent time. I'll get a pull request up in a minute.