trailers are not preserved in responses

[hoodie]

Hi there, great library!
I was reviewing this a little and found that hop-headers get removed from both requests and responses, but only requests put the trailers back in afterwards. I tried to compare this to the Go implementation, it is a bit hard to read, but it seems they also put the trailer back in.

Besides that, I saw that you have some unreleased changes on master. The newer API with the persistent client object is quite nice, do you plan to release this version?

Thank you!