Ticket #6808 Need to exempt json from layouts

Jeremy Kemper committed a bunch of cool JSON stuff, but they are
rendering with the current layout. duh!

Easy fix. Just need to add :json to the hash in candidate_for_layout?.

I have the patch and units tests all ready to go.
http://dev.rubyonrails.org/ticket/6808

shazam! http://dev.rubyonrails.org/changeset/5746

Woohoo, my second patch!

Should of just posted here a week ago.