Ensure exact match for String rules

For instance, `:only => '/account'` won't match
`'/account/login'` anymore (use `:only => %r{^/account}` instead).

@tobmatth I think this behavior is the expected one (as you told me in your email a while back) but since this isn't backward-compatible I didn't to push this change directly to master so we can decide if we want it, and if yes in which version should we include it (probably next major version)...