fix for cast( AS DATETIME)+0 in 5.0 and above versions.
  val_real now works using val_decimal for DATETIME Items
  Superfluous val_real() methods deleted


sql/item_timefunc.h:
  val_real() for datetime functions implemented
      as { return val_real_from_decimal(); }
      It's not a fastest possible way, but code is simple and less
      error-prone, what i belive is more important here as this part
      works unfrequently.