Inconsistent request preparation

[JustAnotherArchivist]

While working on my recent PR, I noticed that the request for uploading is prepared differently when debug mode is enabled. With debugging, it uses the session's prepare_request method:

internetarchive/internetarchive/item.py

Lines 1052 to 1055 in 6e23216

	if debug:
	prepared_request = self.session.prepare_request(_build_request())
	body.close()
	return prepared_request

Without debugging, it uses the request's prepare method:

internetarchive/internetarchive/item.py

Lines 1070 to 1071 in 6e23216

	request = _build_request()
	prepared_request = request.prepare()

I observed header differences between the two, but I can't immediately tell which is more correct, hence why I'm filing an issue rather than a PR. Note that the session.headers do get copied into the request directly inside _build_request. Changing the non-debug code to use self.session.prepare_request breaks several tests, too.

There's another case in Item.modify_metadata where the code directly contradicts the preceding comment:

internetarchive/internetarchive/item.py

Lines 840 to 842 in 6e23216

	# Must use Session.prepare_request to make sure session settings
	# are used on request!
	prepared_request = request.prepare()