Helper method to access controller context in middleware

[NikolayRys]

This is a feature suggestion.

Recently, working with Grape and writing some rescue_from handlers, I've encountered a need to access the request params, headers, and some other stuff how it was in the original controller method.

It was possible, but there was no documented and elegant way to do it. And I needed to go through the Grape source code to figure out a way to reach it.

With this change, we can save effort and time for other clients by preventing the need to rediscover this. It is useful not only for the error handling, but also for every piece of the middleware, including formatters, and any custom middleware inherited from Grape::Middleware::Base.

This PR adds the actual helper method for accessing the context, tests, and documentation for it.

[dblock]

This introduces yet another way to access the same thing, so I think I am against it. I think you want to be able to do params[:x] or request.params[:x] directly like inside any API. What do you think?

[NikolayRys]

It is another way indeed, but:

• The existing way env[Grape::Env::API_ENDPOINT] feels hackish, and is exposing internals to the client.
• I've looked around more. And actually, a similar thing successfully exists in grape middleware already, only for authentication: https://github.com/ruby-grape/grape/blob/master/lib/grape/middleware/auth/base.rb#L14

I'm essentially suggesting to just make consistent what already exists. Now I'm thinking, it would better to put them in a some common module, what do you think?

[dblock]

Ok, I buy this. A module would be helpful, I'd merge that.

[NikolayRys]

Awesome, I'll adjust the PR

[NikolayRys]

As requested, moved it out to a common module.

[myxoh]

Thanks for the hard work

[dblock]

Perfect.