Add specs using keywords as implicit Hash values

[amomchilov]

This documents behaviour like:

def do_something(if:)
  # Behaves like `binding.local_variable_get(:if)`, but faster.
  condition = { if: }[:if]
end

[eregon]

It sounds good to spec that indeed, could you rebase? (since #1187 is merged now)

[amomchilov]

will-do when i have some free time