tag_kwargs are not taken into account when TAGGIT_CASE_INSENSITIVE is True

[dennisv]

tag_kwargs are not taken into account when case insensitivity is turned on, while they are when it is off.
For cases like #809 that would result in already existing tags, with the same name, from other users/tenants being used instead of creating a new one.

When case insensitivity is turned off, this will work as expected as the tag_kwargs are used in the filter to check for existing tags.

In _TaggableManager._to_tag_model_instances:

django-taggit/taggit/managers.py

Lines 221 to 238 in 63c9a97

	if case_insensitive:
	# Some databases can do case-insensitive comparison with IN, which
	# would be faster, but we can't rely on it or easily detect it.
	existing = []
	tags_to_create = []
	for name in str_tags:
	try:
	tag = manager.get(name__iexact=name)
	existing.append(tag)
	except self.through.tag_model().DoesNotExist:
	tags_to_create.append(name)
	else:
	# If str_tags has 0 elements Django actually optimizes that to not
	# do a query. Malcolm is very smart.
	existing = manager.filter(name__in=str_tags, **tag_kwargs)
	tags_to_create = str_tags - {t.name for t in existing}

If I change the get call on line 229 to get(name__iexact==name, **tag_kwargs) it works the same for both cases.