Prefer http_url in HttpsFilter

This updates `HttpsFilter` to prefer the `:http_url` option (and fall back to `:base_url`) to address https://github.com/jch/html-pipeline/pull/131#issuecomment-54685313

/cc @jch @simeonwillbanks 
Yeah, this was a bit awkward, but wasn't sure a cleaner way to do it.