Fix for #211. Updating staging values with nulls now resets those attributes to the defaults in the Cloud Controller

Fix for #211.

In the implementation, I choose not to try modifying Jackson configuration so that null values in Maps are retained in the serialized JSON.   Based on other issues in the backlog, it looked like Spring and Jackson would be updated at some point, which might cause that configuration to change.  Also, the change in configuration may affect all operations in the client.

Please see the code for additional implementation comments.  If there is a better way to have Spring/Jackson send null values, I am open to changing the implementation.