Added support for boolean attribute provided as a string in data source

Added support for `booleanAsStringTestAttribute`. That is, where the desired attribute type is `NSBooleanAttributeType` yet the data source is a string value. 

(This may not be commonly encountered with `JSON`-formatted data sources, but it's very possible if the format is `XML` or `PLIST`).

On the current, latest `develop` commit, this event typically (and in my experience, *always*) resulting in the app crashing during `MagicalRecord` importing.

Also added a test for the new behavior.
All tests passing.