Add support for Transfer-Encoding

- inform server it may use deflate/gzip to transfer encode message
- transfer encodings are listed in the order they were applied

I'm not sure how common it is for servers to use this, although it does sound useful.
Of course, the server should be smart about it.
Compressing a message that's already compressed via content-encoding would be a waste.

Do you think this is worth adding?
If so, should we support this with `:response_block` as well?

Includes #316, #317, #318, #319