Bug #48370  Absolutely wrong calculations with GROUP BY and
  decimal fields when using IF

Added the test cases in the above two bugs for regression
testing.
Added additional tests that demonstrate a incomplete fix.
Added a new factory method for Field_new_decimal to 
create a field from an (decimal returning) Item.
In the new method made sure that all the precision and 
length variables are capped in a proper way. 
This is required because Item's can have larger precision
than the decimal fields and thus need to be capped when
creating a field based on an Item type.
Fixed the wrong typecast to Item_decimal.