Old version of my http implementation.
I no longer work on this, use http2.d instead.
HttpClient keeps cookies, location, and some other state to reuse connections, when possible, like a web browser.
Executes a generic http request, returning the full result. The correct formatting of the parameters are the caller's responsibility. Content-Length is added automatically, but YOU must give Content-Type!
Gets a textual document, ignoring headers. Throws on non-text or error.