Fix for BUG#16676: Database CHARSET not used for stored procedures

The problem in BUG#16211 is that CHARSET-clause of the return type for
stored functions is just ignored.

The problem in BUG#16676 is that if character set is not explicitly
specified for sp-variable, the server character set is used instead
of the database one.

The fix has two parts:

  - always store CHARSET-clause of the return type along with the
    type definition in mysql.proc.returns column. "Always" means that
    CHARSET-clause is appended even if it has not been explicitly
    specified in CREATE FUNCTION statement (this affects BUG#16211 only).

    Storing CHARSET-clause if it is not specified is essential to avoid
    changing character set if the database character set is altered in
    the future.

    NOTE: this change is not backward compatible with the previous releases.

  - use database default character set if CHARSET-clause is not explicitly
    specified (this affects both BUG#16211 and BUG#16676).

    NOTE: this also breaks backward compatibility.