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 
Oops, just read the definition for `http_url`. Ignore this last comment.