fix build_association on 4.2

2 things in this PR,
 - Make tests run up to 4.2, as this wont be tested under master(5.0)
 - Fix build_association bug after rails/rails@f8d2899.

[fixes https://github.com/rails/rails/issues/18121]
[related #46] <- have more context.

review @sgrif @rafaelfranca @guilleiguaran 
also thanks to @JONBRWN and @JGRUBBS for initial implementation.
cc @eileencodes