Add default value testing to bulk tests

This is related to #695 and #676.

The current bulk tests only test the getExplicitValue() method, not defaults. This makes the tests a bit counterintuitive (shouldn't that property never return null??) and also misses an entire aspect of the testing.

There are two flavors of loaders: Trimming and NonTrimming, which leads to two possible explicit values for each property. Adding another response value to test would seem to increase the number of values to test to 4 (2 x 2), however, it is simpler than that. Since the default value is only relevant when the explicit value is null, a single default value can be specified to apply to any null explicit value. Additionally, that it can only be required of the builder when one of the explicit values is null.