Do not send HTTP header if value for header key is None (fixes #473)

May not be the best place for it, and surely there is a nicer way to achieve the same thing.

If you set `headers={"Referer": None}`, you get `{'Referer': [None]}` at `HTTP11DownloadHandler` level.

Probably should be done also for HTTP/1.0

Implementation of https://github.com/scrapy/scrapy/issues/473