Fixed validation of invalid params with mutual_exclusive inside hash

[lasseebert]

This fixes an issue with mutual_exclusive params inside a type: Hash param.

With these params:

params do
  optional :wine, type: Hash do
    optional :grape
    optional :country
    mutually_exclusive :grape, :country
  end
end

And the request params { wine: "This is a string, not a hash" }

I'd expect an error message, but instead get an unhandled error:

undefined method `stringify_keys' for "This is a string, not a hash":String

[dblock]

Can you please squash/rebase these? Thanks.

[lasseebert]

Rebased to latest master

[lasseebert]

...and squashed :)

[dblock]

Merged, thanks.

[lasseebert]

Thanks :)