Change the type of set container depending on the type of key.

[volkanunsal]

Given that the user is setting a value into a deeply nested object, when creating the container elements on the key path, the algorithm should take into consideration the type of the key. Let's take a look at how it works now:

I.setIn({}, ['a', 0], 'foo')
// => { a: { 0: 'foo' } }

The container of 'foo' is assumed to be an object, even though the key is a number. This pull request makes sure when the key is a number the container is an array or a List (if we are in an immutable object.) For example:

I.setIn({}, ['a', 0], 'foo')
// => { a: ['foo'] }