Bugfix to model mommy respect BooleanField default value

This pull request fixes the bug which makes model_mommy doesnt' respect BooleanField value. This bug was not easily catched because of how model_mommy BooleanField generation works choosing randomly True or False value.

With this fix, I needed to remove one test that tries to change BooleanField generator function. As model mommy now respects BooleanField's default value, this test breaks because BooleanField always have a default value, which makes unnecessary to follow with model mommy flow to instanciate the field generator in the type mapping dict.