Optionally force HTTPS content URIs in secure contexts

[joeyparrish]

While debugging a content issue with a partner, we found that the DASH manifest contained plain HTTP URIs. When the page is loaded in a secure context, this causes mixed content errors.

We could add an option that would override http URIs with https URIs when the page context is https. This might be useful for some applications whose packager produces absolute, plain HTTP URIs (as bitmovin seems to do in this case).