Set parameter's type to Item::DECIMAL_ITEM assigning a decimal value.


mysql-test/r/ps.result:
  Fox for bug #28509: strange behaviour: passing a decimal value to PS
    - test result.
mysql-test/t/ps.test:
  Fox for bug #28509: strange behaviour: passing a decimal value to PS
    - test case.
sql/item.cc:
  Fox for bug #28509: strange behaviour: passing a decimal value to PS
    - set Item_param::item_type to Item::DECIMAL_ITEM in case of DECIMAL_RESULT variable.
    - removed redundant item_result_type assignments as it's set before.