Parameterize should accept malformated utf8 strings

When using parameterize I ended up with the following errors:

NoMethodError: undefined method normalize' for "Fort\351":String

This occurred because user data (user name, city name...) sometimes were inserted with malformated characters (something like "Now \251") and parameterize was not able to transform that string into a slug. I assume that parameterize should transform any string into a nice slug, even if it ends up removing characters or generating a slightly different slug from the original string.
So what I did is first verify if the string is a valid utf8, if not it should force the convert to utf8 (with losses if necessary) and then translate to ascii equivalents using Inflector:transliterate.

This should solve this bug too #3504 String parameterize / normalize bug.