Problem: when character_set_connection=utf8,
mixing SPACE() with a non-Unicode column (e.g. for concat)
produced "illegal mix of collations" error.
Fix: Item_string() corresponding to space character
is now created using "ASCII" repertoire. Previously
it was incorrectly created using "UNICODE" repertoure, which
didn't allow to convert results of SPACE() to a non-Unicode
character set.