Fix failing tests in IsUnique rule.
Fix failing tests in IsUnique rule.

A while back the default behavior was changed in 3.2.x to make IsUnique
strict about nulls. When that code was merged into `3.next` I deleted
the 'duplicate' code. Then that change was reverted in `master` leaving
these tests failing. This restores the behavior and fixes the failing
tests.

CakeDC/cakephp,garethellis36/cakephp,HavokInspiration/cakephp,cakephp/cakephp,thinkingmedia/cakephp,tenkoma/cakephp,dakota/cakephp,cakephp/cakephp,nrother/cakephp,ndm2/cakephp,dakota/cakephp,dakota/cakephp,berrygoudswaard/cakephp,cakephp/cakephp,ADmad/cakephp,cakephp-fr/cakephp,tenkoma/cakephp,elboletaire/cakephp,thinkingmedia/cakephp,CakeDC/cakephp,cakephp-fr/cakephp,ADmad/cakephp,petteyg/cakephp,davidyell/cakephp,mvdriel/cakephp,ndm2/cakephp,davidyell/cakephp,ADmad/cakephp,berrygoudswaard/cakephp,tenkoma/cakephp,petteyg/cakephp,davidyell/cakephp,garethellis36/cakephp,cakephp/cakephp,dakota/cakephp,mvdriel/cakephp,cakephp-fr/cakephp,CakeDC/cakephp,ADmad/cakephp,elboletaire/cakephp,mvdriel/cakephp,garethellis36/cakephp,petteyg/cakephp,mvdriel/cakephp,HavokInspiration/cakephp,petteyg/cakephp,garethellis36/cakephp,tenkoma/cakephp,nrother/cakephp,berrygoudswaard/cakephp,HavokInspiration/cakephp,nrother/cakephp,thinkingmedia/cakephp,elboletaire/cakephp,CakeDC/cakephp,ndm2/cakephp,ndm2/cakephp