Support POSIX.1-2016 assignment behavior
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_01
Added some tests in r3727 but no behavioral change is needed for this ticket. #36739 will make some changes in assignment error behavior.
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_01