Bugfix for: ParamValidator ignores req/opt rules when one of them was not specified.
Added a to-do for Solver\Lab\AbstractView.
Changes in trailing slash behavior in Router:

- Trailing slash in canonical URLs (vs. no slash) is now optional.
- This is a B.C. break as the router config now requires a new boolean flag "preferTrailingSlash", but existing code will get an explicit error for the missing flag and can set it to true to achieve the previous behavior.
- File extension detection for "prefer trailing slash" mode is now stricter in order to better avoid false positives.
Added notes to Router regarding trailing slashes vs. no trailing slashes.
Merge branch 'master' of https://github.com/solver/foundation-php