Merge branch 'release/0.20.0-pre6' into development
Fix typo in README.md registration of response descriptor. closes #1125
Fix static analyzer warning about uninitialized `success` variable in `RKMappingTest`
Add another mention about the objectClass in RKRequestDescriptor
Add helpful error message if you call `requestMapping` on an `RKObjectMapping` subclass
Pull the CoreData header into the PCH to trigger conditional compilation of Core Data specific bits. refs #1126

This is a distateful temporary workaround until I can find a better solution. To try and decouple components from Core Data,
I have introduced conditional compilation against the `_COREDATADEFINES_H` value. This enables you to opt into the Core Data specific
features by adding the import into your target's PCH file. Unfortunately at the moment on CocoaPods, there is no way to add the import
to the PCH only when you pull in the Core Data subspec. I have opened an issue on CocoaPods to pursue a solution.

See https://github.com/CocoaPods/CocoaPods/issues/723
Drop unnecessary `performBlockAndWait:` invocation that may be correlated with occasional crashes seen in GateGuru
Add a more informative assertion when an `RKManagedObjectResponseMapperOperation` instance is not configured properly
Fix test breakage when your ISP returns a custom HTTP response page for an invalid host
Bump OCHamcrest checksum
Fix issue where unsaved managed object POST'd to the server with unsaved changes are not persisted if no mapping occurs. fixes #1130
Add support for customizing the `RKObjectRequestOperation` and `RKManagedObjectRequestOperation` instances built through the object manager using a registration API very similar to that of `AFHTTPClient`. closes #1131
Add new `RKManagedObjectStore` method for assisting in performing migrations when you have made dynamic modifications to the managed object model (such as when you leverage the Search indexing support)
Unify `RKHTTPRequestOperation` and `RKObjectRequestOperation` subclass customization via registration API's. closes #1131
Fixed issue when body could be null on device

stringWithUTF8String accept NULL terminated C array of UTF8-encoded
bytes.
But NSData stores encoded data not like NULL terminated string.
Guard against crash due to Nil value retrieved for managedObjectClassName in the property inspector
Use a visitor to identify all `NSManagedObject` instances being sent via the object manager which have a temporary managed object ID to avoid crashes during mapping. refs #1001
Remove accidentally committed debug logging
Fix issues with implementation of `error` override in `RKHTTPRequestOperation`. fixes #1140
Merge branch 'master' of github.com:AntonPalich/RestKit into development