Append trailers to the existing headers
Append trailers to the existing headers

This is imperfect because you can't tell what is a trailer and what is a
header, but it solicit from choking when a HTTP/2 servers sends
trailers.

Previously solicit was overwriting the existing headers, and then
declaring the stream Malformed when it was unable to find the :status
header.

stepancheg/solicit,mlalic/solicit