Record deserialization when encountering un-deserializables

[cheungpat]

follows #64

The iOS SDK expects the server to send valid response. In case the server sends a response that the iOS SDK does not expect, SKYDataSerialization returns nil (which IMHO is the expected behavior). This caused nil being added to NSDictionary, which is illegal.

The SKYRecordDeserializer should, on encountering a nil value, do one of the following (your pick):

• set [NSNull null]
• not set the key
• ~set SKYUnknownValue~ maybe not this one

and also emits a log stating that the value of the key failed to deserialize.

[cheungpat]

i wonder if you have time to do this, should be a simple fix @ben181231