Change `validate_uniqueness_of_matcher` to give non-null columns default values

Previously, if a model had an attribute that mapped to a non-nullable database
column that was not under test the matcher would fail with an SQL error like:

    examples.column may not be NULL #...

With this change the matcher attempts to provide a default value for all
non-nullable, non-primary attributes. This should avoid the above error in
simple cases.