Fix mutable default argument values

[g-nie]

Hey there, I noticed some function arguments using mutable types (sets, dicts) as default values.
While this doesn't appear to cause issues right now, it could lead to unexpected behavior if any future code modifies these default objects.
I thought to change the defaults to None and initialize them in the __init__s if needed. What do you think?

[blag]

Absolutely needed.

There are alternative ways to do this, but the way you chose works perfectly fine.

[blag]

Can also do this:

Suggested change

	if attributes is None:
	attributes = {}
	if clean_content_tags is None:
	clean_content_tags = set()
	if tags is None:
	tags = set()
	attributes = attributes or {}
	clean_content_tags = clean_content_tags or set()
	tags = tags or set()

[blag]

Same suggestion as above, just adding it to make your life easier if you like this better:

Suggested change

	if attributes is None:
	attributes = {}
	if clean_content_tags is None:
	clean_content_tags = set()
	if tags is None:
	tags = set()
	attributes = attributes or {}
	clean_content_tags = clean_content_tags or set()
	tags = tags or set()