WRONG VALUES

User variables will have the default session collation
associated with it. And a select which uses it as part of a
union may infer the collation while type merging.
This leads to problems when the result is of DECIMAL type.
Setting the appropriate collation of DECIMAL result type
is missing in 5.7 code base.

Added code to set appropriate collation when the result is
of DECIMAL type during Item_type_holder::join_types().