I pulled `content_md5` and `content_type` into the arguments list, with defaults of None.

I want to keep the logic of "what is GCS's correct representation of `None` in the API" in the method, not in the default arguments. So:

    def generate_signed_url(self, resource, expiration, method='GET', content_md5=None, content_type=None):

seems like a better option, and I'd like to keep the ORs.