Merge pull request RestKit#83 from sixten/0.9-error-handling

Fix for crash based on set nil for missing element policy. Added spec…

…s covering the crash. Merging directly to 0.9.1 and tagging as 0.9.1.1

Merge branch 'master' into 0.9

Silenced a couple of Xcode 4 warning. Don't send a serialization with…

… a GET request by default