Add Support for Nullifying Delegates for Requests about restkit

[grgcombs]

Commit 20fb904 deals in part with this.

As per IRC discussions:

If NSURLConnection's stop sending any further information to delegates upon receiving a cancel, then canceling in an RKRequest presents problems when requests can be cancelled by the backgrounding support. If a cancel nils the delegate (or equivalently orphans it) then you would get a resent request without a delegate.

So therefore, we should consider breaking apart the functionality in RKRequest ... put the NSURLConnection's cancel in a separate method that clearly indicates permanent destruction of the connection and orphaning of the delegate. In other words, we shouldn't stick this in the same group of calls with an RKRequest's reset or cancel without some caveats and warning bells that all hell is formulating a plan to break loose.

Granted, this sort of behavior of nullifying delegates or not can change at a moment's notice whenever Apple modifies the SDK or underlying device software, and the documentation isn't perfectly clear. More testing is probably necessary before investing a great deal of time in workarounds.

[blakewatters]

Wontfix. RKRequest is now eliminated so we don't need to address.