type annotation of "headers" argument of scrapy.Request is incomplete

[kmike]

Currently scrapy.Request(headers=...) argument is annotated as Optional[dict]. But we allow other options, e.g. a list of (name, value) pairs, which allow to set multiple headers with the same name. Headers instance may also be an option for the users to pass. I think we should ensure that none of this raises mypy typing errors.

[wRAR]

Correct and full typing of CaselessDict and Headers doesn't seem possible, so I think I'll just document the interface and silence the incompatibilities where needed.