Fix bug and implement return type casting.


mysql-test/r/sp.result:
  Bug#9775
    Test for bug + feature
  Fix previous tests
mysql-test/t/sp.test:
  Bug#9775
    Test for bug + feature
  Fix previous tests
sql/item_func.cc:
  Bug#9775
    new method Item_func_sp::execute(Field **)
    some optimizations.
sql/item_func.h:
  Bug#9775
    results for Item_func_sp pass through a Field of the return type.
    new method Item_func_sp::execute(Field **)
sql/sp_head.cc:
  Bug#9775
    missing initialiation for type_lengths in sp_head::create_typelib()