Add missing leading slash when canonicalising

This means that for a path-like input such as:
  first-level/second-level

we no longer trash it to make:
  /second-level

we instead make it into:
  /first-level/second-level

On the other hand it does mean that other things accidentally pasted into a
paths field will be turned into paths...
Merge pull request #367 from alphagov/add_missing_leading_slash_when_canonicalising

Add missing leading slash when canonicalising