it happens because of the LEFT JOINT optimization in add_key_part()
This optimization does exactly the same in JOIN and in WHERE conditions 
Not right.
I moved that optimization one level upper.


mysql-test/r/join_outer.result:
  appropriate test result
mysql-test/t/join_outer.test:
  test case
sql/sql_select.cc:
  optimizing of the left_join now only for WHERE conditions
  and for first table in list