HTTP2 outgoing messages should set content-length if payload knows it

[seanmonstar]

For instance, a server replying with Response::new(Body::from("Hello, World!")) over HTTP2 should set the content-length: 13 header.

Steps to fix

• In proto::h2::server, before calling SendResponse::send_response, check body.content_length(). If it returns Some(len), check if the headers contain CONTENT_LENGTH, and if not, add the header with the serialized length.
• Do the same in proto::h2::client, before calling SendRequest::send_request.
• Serializing into a header value can make use of hyper::headers::content_length_value.
• A new function, set_content_length_if_not_present, could probably be added to hyper::headers, making use of HeaderMap::entry.