Allow clients to indicated that paths in URLs need not be quoted

I was dealing with a server which only accepted something like `http://some.server/dGVzdCBjYXNlPQ==`, but not `http://some.server/dGVzdCBjYXNlPQ%3D%3D`. And the server is out of my control, so I can only work on it on the client side.

I was thinking it would be good to have the client offload the path-quoting operation to it's calling code too, so one more keyword parameter `quote_path` is added to `aiohttp.request(...)` and `ClientRequest.__init__(...)`