SI-7837 quickSort, along with Ordering[K], may result in stackoverflow because the code uses '==' instead of 'equiv'

== instead of equiv (from Ordering) was used by mistake.  Fixed.

Also created a test to make sure that == is not used by throwing an exception if it is (as suggested by Jason).
Merge pull request #3301 from Ichoran/issue/7837

Resolves SI-7837, failure in quickSort.