fix(INNamespace): respect resourceMapping when updating threads and tags

Previously, when merging properties into objects being updated, the
properties were not transformed into client-side properties (defined in the
resourceMapping for the model). This CL provides simple machinery for
ensuring that this works as expected.

In addition, machinery is added for merging simple arrays, such as
messageIDs or draftIDs.

Still TODO: support correctly merging participants and tag data arrays
during update.

Closes #30