Change to SameValueZero for comparisons

[jdalton]

Currently for comparisons in methods like difference, includes, indexOf, intersection, lastIndexOf,uniq, union a strict equality, === is used.

If this was tweaked a bit to SameValueZero, === + NaN matches, then it would allow for optimizations with Set and Map as well as cover the NaN match case.

Thoughts?

[jdalton]

It would mean:

_.indexOf(['a', NaN, 'b'], NaN);
// => 1 instead of -1