Send headers and body in one write if possible
Send headers and body in one write if possible

This change tries to avoid a pathological case with Nagle's algorithm where two
small writes that would fit in a single TCP packet are performed in a row
(headers and body).

Instead, it tries to perform a single write if possible.

Connorhd/http.rb,httprb/http.rb,httprb/http,pezra/http.rb