Rack rack

In web apps, we've been using the PUT verb improperly, according to the official specs. The PUT verb should be used to send back a "replacement document", not only a few pieces. But most of the time we only send back a few attributes (for instance, in Rails, we usually don't have the 'created_at' or 'updated_at' fields in the form to send back in an update).

Fortunately, the PATCH verb has been finally approved, and this is the correct verb when we just want to update some of the attributes of a given document (or model). You can refer to RFC 5789 (http://www.innoq.com/blog/st/2010/03/rfc_5789_patch_method_for_http...) for the details on the approval.

As such, I thought it would be good to have Rack already supporting this verb, so I hacked together a quick patch to add it. Please let me know if I did something wrong.