Fix for lp:780425 sql_buffer_result=1 gives wrong result for GROUP BY with a +# constant expression"

mysql-test/r/select.result:
  Test case for lp:780425
mysql-test/r/select_pkeycache.result:
  lp:780425
mysql-test/t/select.test:
  lp:780425
sql/sql_select.cc:
  Added DBUG_ASSERT to be prove some logic and later be able to simplify the code
  Set implicit_grouping if we delete a GROUP BY to signal do_select() that a grouping needs to be done.