The code that set up data to be passed to user-defined functions was very
old and analyzed the "Type" of the data that was passed into the UDF, when
it really should analyze the "return_type", which is hard-coded for simple
Items and works correctly for complex ones like functions.
---
Added test at Sergei's behest.


mysql-test/r/udf.result:
  Verify that various arguments work.
  ---
  Added test at Sergei's behest.
mysql-test/t/udf.test:
  Verify that various arguments work.
  ---
  Added test at Sergei's behest.
sql/item_func.cc:
  For function-Items, test whether it is constant and set the struct members 
  for the UDF parameter appropriately.
  
  Replace tabs with spaces in affected code.
sql/udf_example.c:
  Include a simple function that is useful in testing.