Added item.real_type() for easy access to the underlaying types for Item_ref and Item_cache_wrapper()
This allows us to simplify and speed up some tests and also remove get_cached_item()

sql/item.h:
  Added item.real_type()
  Removed get_cached_item()
sql/opt_range.cc:
  Simplify test
sql/sql_select.cc:
  Simplify test
sql/sql_show.cc:
  Simplify test