improve performance of StringUtil.replace()

if the "find" is not found in input then there is no reason to create a StringBuilder and return a new string object.

this change makes `Replace(input, find, replace)` more performant than

    if (Find(input, find))
      input = Replace(input, find, replace);

do you have a testcase for function replace()?  I don't see one in railo-tests/functions