Add support for setting cookies for individual request

[p0lm]

Long story short

A cookies kwarg that can be set with individual request providing cookies just for a single time.

Expected behaviour

Aiohttp provides an interface that mimics requests, but lacks support for sending a dict of cookies with a single request. It's be nice to have an optional keyword argument cookies on sending a request, providing a way to pass in a dict of cookies that are added on top of the cookies stored in the cookiejar.

[asvetlov]

Well, it would be good feature.
Would you provide a Pull Request?

[p0lm]

I'll try to take a swing at it later this week. It seems not too hard to implement.

[blueyed]

This can be closed, #3396 was merged.