Ensure mb formatString is congruent to standard ucwords() functionality

Currently the mb title case change is not congruent with uc_words() in that it ignores leading numeric characters. This leads to annoying addresses like "555 1St Ave" instead of "555 1st Ave". This small patch fixes that problem and is still unicode safe.
@willdurand Is it safe to assume if mb_convert_case exists that the other mb functions will exist? I can probably remove this redundant check, I think.