Route pattern doesn't support trailing optional

[paulmelnikow]

As mentioned in the rewriting services doc:

At present, the trailing parameter of a pattern can't be optional. If the last part of a route is optional, like a branch, you will need to use a format regex string (e.g. format: '([^/]+/[^/]+)').

Have got an issue open upstream in pillarjs/path-to-regexp#173, though it's not clear how that will be resolved and it's worth having a place to track this here.