handling of duplicate keys in Object$prototype$concat

Sanctuary:

> Z.concat({x: 1, y: 1}, {y: 2, z: 2})
{x: 1, y: 2, z: 2}

Haskell:

> M.fromList [('x', 1), ('y', 1)] <> M.fromList [('y', 2), ('z', 2)]
fromList [('x', 1), ('y', 1), ('z', 2)]

We didn’t discuss this behaviour in #2 as far as I can see; I think we diverged from Haskell’s behaviour unintentionally.

/cc @puffnfresh

[davidchambers]

Let’s stick with the current behaviour for the time being. As it’s not clear that one approach is better than the other it’s best to leave things as they are. We can revisit this issue if new information comes to light.

[safareli]

In haskell mappend of Map is “left-biased” in JS we have similar function Object.assign (R.merge has similar semantics) which are right biased, so I think is better to stick with what we have as it’s more intuitive to JS developer.