determine canvadocs http/https based on url

This is common functionality, so I pulled it out into the canvas_http
gem.

test plan: a canvadocs config using an http:// url should work, as well
as https:// urls which already worked.

Change-Id: I640445bfcf9b5c903d37aeb9db6c566430e2113d
Reviewed-on: https://gerrit.instructure.com/36760
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>