Adding test
item_sum.cc:
  Adding a call for collation/charset aggregation,
      to collect attributes from the arguments. The actual bug fix.
item_func.h, item_func.cc, item.h, item.cc:
  - Removing collation aggrgation functions from Item_func class
      in item.cc, and adding it as non-class functions in item.cc
      to be able to reuse this code for group_concat.
      - Adding replacement for these functions into Item_func class
      as wrappers for moved functions, to minizize patch size,