register_type does not work with subclasses of str

[realsuayip]

This is probably true for all primitive JSON types, I only tested str. Take this example:

from django.utils.safestring import SafeString, mark_safe
from kombu.utils.json import register_type

register_type(SafeString, "safestring", str, mark_safe)

This does not seem to work as SafeString is a subclass of str.

I'm not sure how this could be fixed. If nothing, we should probably document this.