Let photon handle https images.

Closes #1808 
Includes a new test.
cc @koke cos I recall you were interested in photon supporting https. 
A URL without a scheme would have trouble with `url.scheme.length + 3` depending on if it included the :// or not. You make a good point about `://` appearing elsewhere in the URL, like in the query string.  I'll add a sanity check for the location of the match.