Respect default scope["actor"] if one exists

[simonw]

ASGI wrapper plugins that themselves set the actor scope variable should be respected (though actor_from_request plugins should still execute and get the chance to replace that initial actor value).

Relevant code:

datasette/datasette/app.py

Lines 910 to 921 in 09a3479

	# Handle authentication
	actor = None
	for actor in pm.hook.actor_from_request(
	datasette=self.ds, request=Request(scope, receive)
	):
	if callable(actor):
	actor = actor()
	if asyncio.iscoroutine(actor):
	actor = await actor
	if actor:
	break
	scope_modifications["actor"] = actor