Allow InputObject instances to be pattern matched via #deconstruct_keys

[czan]

This allows Ruby's case/in syntax, like:

case input
in { x:, y: }
  # do something with x and y
end

This is easy for individual projects to add in their own InputObject superclass, but it seems generally useful, so I figured I'd raise a PR with it. No hard feelings if you don't want it upstream. 🙂

[rmosolgo]

Thanks for this improvement!