Merged r12070 from trunk to 2.3-stable
not use assert_not_nil in Errors#[].
r7593 etc. replaced Rails2 Errors#on. Rails3 Errors#[] always return array. So, Rails3 Errors#[] is always not nil.
Authored by: tmaruyama 2013-07-30
Parent: [r12070]
Child: [r12072]