Author: Zig Tan
MDL-38301 mod_assign: Fix date validation logic & message consistency
http://git.moodle.org/gw?p=moodle.git;a=commit;h=8f7d07fa7d6a171a5396fbaf69be1be38771d7f5
http://github.com/moodle/moodle/commit/8f7d07fa7d6a171a5396fbaf69be1be38771d7f5
+ 3.6 en [cutoffdatevalidation,mod_assign]
+ 3.6 en [cutoffdatefromdatevalidation,mod_assign]
+ 3.6 en [duedatevalidation,mod_assign]