Simplify parametersString by directly using is expression

While writing the unittest for #870, I realized that even Phobos was bugged w.r.t to this (e.g unnamed parameters would have produced CT errors, rather than hit https://github.com/rejectedsoftware/vibe.d/blob/master/source/vibe/web/rest.d#L524 ).

A more general and simpler solution was to re-implement `parametersString` in terms of `is()` expression, as the various `Parameter*Tuple` are just breaking them down.