:if should not fire on validations when not in context with :on

[asanghi]

Related to Issue#341

#341

validates :title, :presence => true, :if => Proc.new do |x|
    #expensive action
 end, :on => :update

The validation should not call the Proc above during :create

The bug above states that as the problem. This patch fixes it.

/cc @josevalim @jwkpiano1

[josevalim]

Merged!