On systems < iOS 5, which do not support zeroing weak references, the
requestDidFinish delegate method will be called on the delegate after it
has already been releaed by the system. This works fine with zeroing weak
references, where a method call on nil will simply be ignored.
This poses a problem, however, if the reference in question has never been
zeroed out. The solution is to remove the in requestDidFinish on non
weak reference systems.