Fix the problem introduced by previous refactor in shallowEqual

[jddxf]

Previous refactor shallowEqual has introduced an issue as follow

function A() {}
A.prototype.a = 1
const a = new A()
const b = { a: 2 }
shallowEqual(a, b) // returns true, expects false

[timdorr]

This will be handled by #591. Thanks!