time per connection
  Removed const_string() method from Item_string (it was only used in one
  place, in a bad way). Defer possible SP variable, and access data directly
  instead, in date_format item.


mysql-test/r/sp.result:
  Updated results for new test (BUG#17476).
mysql-test/t/sp.test:
  New test case (BUG#17476)
sql/item.h:
  Removed const_string() from Item_string.
  It was only used in one place, and we can just use str_value in Item directly.
sql/item_timefunc.cc:
  Must defer a (possible) local SP variable to use max_length and str_value
  in Item_func_date_format::fix_length_and_dec(), and refer to str_value
  directly without the const_string() method (now removed); the cast didn't
  work in all cases anyway.