Support to_s during escaping

Reported by Jonas Bähr

In my templates I have many calls to the latex_escape function with objects that are not necessarily Strings (no :gsub). The old version of rtex used to call :to_s for each object before the gsub'ing so there where no problems. The following patch restores this behaviour. It would be nice if you could integrate it since it prevents a lot of porting and keeps the templates more readable.