add ability to disable jetpack's https check for photon

[willnorris]

Today, jetpack's photon feature will not work for image URLs that have an 'https' URL scheme. This is presumably because photon URLs don't specify the URL scheme for the original image, so defaults to using 'http'. However, in the case where the http URL redirects to the equivalent https URL, photon (the service) works just fine. to wit:

original photo: ​http://willnorris.com/content/uploads/2012/12/pocket-share-to-google.png (this will redirect to the same URL with https)

photon photo: ​https://i0.wp.com/willnorris.com/content/uploads/2012/12/pocket-share-to-google.png?resize=100,100

The jetpack plugin, however, won't even attempt to replace https image URLs with the photon equivalent, since it has no way of knowing whether the necessary http->https redirect is in place. The provided patch here adds a new filter which allows the strict https checking to be disabled for those site owners who do have the necessary redirect in place. The default behavior of skipping https URLs remains intact.

original support discussion: ​http://wordpress.org/support/topic/photon-and-https-urls