HashMap union behaviour changed in 15.1

[franziskuskiefer]

With the change in #163 the behaviour of union changed and the documentation is wrong now. It's unfortunate that such a breaking change is done in a minor version.

The comment says

    /// Construct the union of two maps, keeping the values in the
    /// current map when keys exist in both maps.

This is not the case anymore when the first map is the shorter one.
Adapting the example from the function description, the following fails

let map1 = hashmap! {1 => 1};
let map2 = hashmap! {1 => 2, 2 => 3};
let expected = hashmap! {1 => 1, 2 => 3};
assert_eq!(expected, map1.union(map2)); // the result is actually {2: 3, 1: 2}

Is this change in behaviour intentional? If it is intentional, the documentation should be changed and the new behaviour should be made clear in the release notes.