Changed another unecessary use of Item_string into LEX_STRING (in sp_pcontext).


Docs/sp-implemented.txt:
  Added IF EXISTS to DROP PROCEDURE|FUNCTION
mysql-test/r/sp-error.result:
  Added IF EXISTS to DROP PROCEDURE|FUNCTION
mysql-test/t/sp-error.test:
  Added IF EXISTS to DROP PROCEDURE|FUNCTION
sql/sp_pcontext.cc:
  Changed another unecessary use of Item_string into LEX_STRING.
sql/sp_pcontext.h:
  Changed another unecessary use of Item_string into LEX_STRING.
sql/sql_parse.cc:
  Added IF EXISTS to DROP PROCEDURE|FUNCTION
sql/sql_yacc.yy:
  Added IF EXISTS to DROP PROCEDURE|FUNCTION