Cannot perform diff on a Map with non string keys

    > a = i.Map().set(null, 1)
    > b = i.Map().set(null, 2)
    > id(a, b)
    param str (null) is not a string

The error comes from https://github.com/intelie/immutable-js-diff/blob/master/src/path.js#L14 where it is trying to determine the path of the key (https://github.com/intelie/immutable-js-diff/blob/master/src/diff.js#L56).