flatMap (and possibly others) sends strings for index

[rtfeldman]

Immutable(["foo", "bar"]).flatMap(function(val, index) {
  return "index is " + index + " aaaand its type is " + (typeof index);
});

// [ 'index is 0 aaaand its type is string', 'index is 1 aaaand its type is string' ]

The offending line of code: https://github.com/rtfeldman/seamless-immutable/blob/master/src/seamless-immutable.js#L124

Evidence suggests that second argument needs to be parseInt(key) which is absolutely gobsmacking.

[rtfeldman]

This is now fixed more efficiently by af6a8df