_.union doesn't work with arrays of objects

_.union will always produce duplicates when passed arrays of objects.

e.g. `_.union( [ { a:1 } ], [ { a:1 } ])` will return `[ { a:1 }, { a:1 } ]`

Perversely, underscore's own isEqual function will tell you that the objects in question are equal. Maybe we could have a flag/option which dictates the equality comparison to use, or the option to pass in a comparator?

Reproduce the example with _.union and compare its behavior with _.isEqual; no source file or test is named in the issue. Before changing anything, resolve whether object equality or a caller-supplied comparator is intended, then add regression coverage for the agreed behavior.

Written by the indexing model from the issue text.