json & jsonb constraints support

[marocchino]

Hi, I want some constraints to json just like this.

Unfortunately, current version can not receive something like jsontable(((j->>'uuid')::uuid)) because of this wrapper.

rein/lib/rein/util.rb

Lines 17 to 21 in f529894

	if /^".*"$/.match?(attribute)
	attribute
	else
	"\"#{attribute}\""
	end

If you don't mind, I want make PR for it. how do you think?

[nullobject]

Sure, if you're happy to make a PR please go ahead 😄