Fixed an issue where a class is dependency injected with a nil value object specifically with bindBlock otherwise an exception setObjectForKey: object cannot be nil (key: propertyNameToBeInjected) is returned due to the propertiesDictionary in use. Added tests for testing that a class dependency injected with a property using objection_requires can be set to nil.
Merge pull request #39 from mpurland/master

Ensure that when an injector context returns a nil object that it is properly mapped to an NSNull before being set on the target object.