test(threads.spec.js): fix spec description

Test case was essentially copy/pasted from namespace.spec.js, and was never renamed. Fixed now.
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
Closes #32