HTTP2 should strip connection headers before sending

[seanmonstar]

Connection-specific headers are forbidden in HTTP2, as that information has been moved into frame types of the h2 protocol. hyper should strip those headers before sending a request or response over HTTP2.

Steps to fix

• Update proto::h2::strip_connection_headers to inspect the value of the CONNECTION header, and remove all headers it references. See this code for how.
• Also remove other headers that h2 has determined are "connection" headers, such as those in this code.

[joshleeb]

I'll start taking a look at this one 😄