fix(INThread): updating via INThread#messages() now respects resourceMapping

3f004b8823a45544f007caae293da3ce7ebbb1fc adds support for mergeArray to respect resourceMappings of
a resource type when merging. This was previously enabled for INNamespace#threads() and
INNamespace#tags(), this CL enables this feature for INThread#messages() as well.

Closes #36
test(message.spec.js): add infrastructure for testing INMessage models

Also adds a test case for the preceding commit in this same CL, in order to demonstrate that updated
messages via INThread#messages() will respect INMessage.resourceMapping.