Add unit tests for readAuthTask and writeAuthTask

Add unit tests for readAuthTask and writeAuthTask.  Please note that I added a fakeSchema to run these tests.  This is an ugly solution because I couldn't figure out a way to extend UserSchema dynamically.  I will remove this fakeSchema once I figure out a way to extend UserSchema dynamically.

Also, I fixed a minor bug in readAuthTask and writeAuthTask methods to retrieve auth task subdocument correctly.

@krulwich, @suprbh, @ceg1236 - please review.