Support initializing RLMObjects using kvc objects without getters

Closes #1236

@tgoyne @jpsim 
So we return nil rather than throwing an exception when trying to initialize an object with an object of another type which is missing a key, but where a default value is specified for the object being initialized. We don't necessarily have to do this, but it is an example of what users will have to do in this situation. All this came about from a user trying to create an RLMObject from a Parse object. Can you think of a situation where this is undesirable?