topdown: Make http.send() caching use full request

Previously it just used a string key with the method and url. This
causes problems if headers or body contents affect the response, and
users were seeing cached responses returned for different requests.

The new version takes into account the entire parameters object that
is provided to the builtin function.

Fixes: #1980
Signed-off-by: Patrick East <east.patrick@gmail.com>