mappings_controller whitespace
Canonicalize mapping filters

* Canonicalizes filter input
* Munges strings to "pretend" to be a path if just a substring
* Does not canonicalized new_url filter
Deal with scheme on new_url case
When not c14n'ing filter (as with new_url), just extract path

* Was returning whole URL with / prepended
Make much of CanonicalFilter private
Reveal intent when canonicalizing a plain substring
Improve readability of CanonicalFilter:

* Shorten cases and don't combine return
* Add a better comment to the class