Merge: String: Bugfixes

When rewriting the to_s method in Array, an error was subtle enough to fight its way through the tests but was discovered when reworking the Ropes (yup, be prepared, it'll appear once more).

Oh and there was a bug in the substring_from method (well, more of a lack of efficiency then a real bug).

Pull-Request: #699
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>