Query string passthrough

[akx]

This PR addresses the querystring mutilation part of #3:

• The default get_upstream_http_options() implementation no longer sets params.
• Instead, it calls the new get_upstream_url_with_query function whose default implementation appends the verbatim client querystring to whatever the user-defined get_upstream_url() function returns.

cc @jokull – would you like to take a look at the implementation? I think this shouldn't break your current code either.

[jokull]

LGTM!

[ruksi]

less params, more new and shiny get_upstream_url_with_query and no more over-massaging of URL query strings

and the header fiddling left as an exercise for another day 👍

lgtm 🚢