was erroneously converted to double, while the result of
ROUND(<decimal expr>, <int literal>) was preserved as decimal.
As a result of such a conversion the value of ROUND(D,A) could
differ from the value of ROUND(D,val(A)) if D was a decimal expression.

Now the result of the ROUND function is never converted to 
double if the first argument is decimal.  


mysql-test/r/type_decimal.result:
  Added a test case for bug #28980.
mysql-test/t/type_decimal.test:
  Added a test case for bug #28980.