Composite types are not compatible with Django serializers

[georgebrock]

Using django.core.serializers to serialize model objects that use composite types (e.g. by running python manage.py dumpdata) will result in data that cannot be deserialized (e.g. by running python manage.py loaddata).

This could probably be fixed by adding a postgres_composite_types.BaseField.value_to_string method that returned a string that was acceptable to Postgres (e.g. for a type composing two integers a string like "(1,2)" would work).

[georgebrock]

@timheap Thanks for the fix!