Fix keyword argument handling

[jochenseeber]

Fixes FallbackContextProxy to fully support 2.7 keyword arguments.

I kept that the different signatures for pre-2.7 und 2.7+ are currently created using stringified methods (class_eval). I would suggest not creating the methods using strings. The resulting duplication of some code would be better than hiding code in strings, IMO. If ok I would create an additional PR for that.

[jochenseeber]

I'll look into the CI failing in the next days. Is there a way to run these checks locally?

[ms-ati]

@jochenseeber It looks as though perhaps the "inline" passing of the start of the << as a parameter is not supported before Ruby 2.3. Would it be possible to try defining the entire string first, and then passing the string to class_eval as a second step?

To test in Ruby 2.2, is it possible to simply rvm install the earlier Ruby version locally? If not, perhaps you might try https://github.com/ms-ati/docker-rvm ?

[ms-ati]

This is superceded by #52 so I am closing -- please let me know if I've misunderstood! And thank you very much for your work to fix this issue!