preValidate method cannnot handle conditional validation.

[kochizufan]

As README says in
https://github.com/thedersen/backbone.validation#do-you-support-conditional-validation
Conditional validation is allowed, but in preValidate method, it cannnot work.
So I make patch for it.

[cmcrothers]

I had the same problem and can confirm that this patch works.

I also created a similar solution but without introducing additional parameters to the method

[f095b48]