Factor out basic authentication headers calculation

[nijel]

Expected behaviour

There should be a single place to create HTTP authentication headers from KEY / SECRET.

Actual behaviour

There seem to be at least 7 implementations of this.

What are the steps to reproduce this issue?

Look around the code for code blocks like:

social-core/social_core/backends/chatwork.py

Lines 27 to 31 in 3d95a29

	def auth_headers(self):
	return {
	"Authorization": b"Basic "
	+ base64.b64encode("{}:{}".format(*self.get_key_and_secret()).encode())
	}

social-core/social_core/backends/beats.py

Lines 24 to 31 in 3d95a29

	def auth_headers(self):
	return {
	"Authorization": "Basic {}".format(
	base64.urlsafe_b64encode(
	"{}:{}".format(*self.get_key_and_secret()).encode()
	)
	)
	}

Any other comments?

Probably the base class should get something like get_key_and_secret_basic_auth and it should be used in subclasses. Maybe even there could be a BaseOAuth2 subclass implementing auth_headers this way.