Null Key Handling

Added support for configuring whether null keys should be allowed after translation.

Part of #37.
Translate Maps with Key Source

If a Map Property has a keySource attribute, the map should be translated rather than simply copied.

Part of #37.
Test for KeySource-Only Translations

Test for translations where the key is translated via a key source expression and neither the type nor the value is translated.

Part of #37.