Log Message:
Changes 5.5.12:
* Fixed bugs: Illegal mix of collations
* Problem: comparison of a DATETIME sp variable and NOW() led to Illegal mix of
  collations error when character_set_connection=utf8.
* Error happened in Arg_comparator::set_compare_func(), because the first
  argument was errouneously converted to utf8, while the second argument was
  not.
* Fix: separate agg_arg_charsets_for_comparison() into two functions:
  - agg_arg_charsets_for_comparison() - for pure comparison, when we don't need
    to return any string result and therefore don't need to convert arguments
    to @@character_set_connection:
          SELECT a = b;
  - agg_arg_charsets_for_string_results_with_comparison() - when we need to
    return a string result, but we also need to do comparison internally:
          SELECT REPLACE(a,b,c)
    If all arguments are numbers:
          SELECT REPLACE(123,2,3) -> 133
    we convert arguments to @@character_set_connection.