Bug 834422, og:image should include a protocol

Facebook doesn't accept protocol-relative URLs in the `og:image` property. Simply replace `//` with `https://` with the new `absolute_url` filter.

My previous commit https://github.com/kyoshino/bedrock/commit/d342519d8ef3819f4fa7ed5f6efb44dc15353951 was adding an option to `media()` to include a protocol, but I thought it was unnecessary here.