Allow null values in dict-ized RemoteObjects

[mmalone]

RemoteObjects currently exclude fields with a None value when objects are converted to dictionaries. This can be problematic for APIs that don't treat nonexistent keys and null values the same way.

Perhaps we should provide some mechanism that allows the "default" value of a field to be an AttributeError, then include all values in the dictionary (including None) only skipping those that raise an AttributeError.