Extract handle_method_call from method_missing

[wasabigeek]

Continues the refactoring started in #549 on the road to better keyword args matching. This is part of a refactor sketched out here.

The key change is to pass arguments and the block directly instead of re-splatting them. That way, we would only need to set ruby2_keywords at the edges, instead of with each splat (for contrast, see initial spike, where ruby2_keywords had to be set at multiple layers).

The extracting of handle_method_call is not strictly necessary, but it makes it clearer that method_missing is just one of the possible entry points for a method call.

[floehopper]

@wasabigeek

This looks good to me 👍

For some reason CircleCI didn't seem to run a build for this PR, so I ended up pushing up a tracking branch to trigger this build. I also took the opportunity to amend the commit note to include the notes from the PR description.

Merged manually in 6a09a37.