Don't strip out trailing slashes when filtering

* Fix #68376922
* Or more specifically, put trailing slashes back after canonicalising

An example of the original problem:
“When trying to filter path on /sta/, it filters on /sta. As a result,
"/statistics" ends up included in my "/sta/" mappings list, when all I
want is /sta/ things for the Standards and Testing Agency”
Merge pull request #255 from alphagov/filter-with-trailing-slash

Don't strip out trailing slashes when filtering