Fix bug a elements are not deleted at index 0

[wadackel]

Hi @georapbox, Thank you for Good Libraries 👍

ImmutableDelete does not seem to behave as expected when deleting an element with index 0.

const arr = ['a', 'b', 'c', 'd', 'e'];
const res = immutableDelete(arr, 0);

console.log(res);
// Result: ['a', 'b', 'c', 'd', 'e']
// Expected: ['b', 'c', 'd', 'e' ]

This PR fixes these bugs.
Thanks.

[georapbox]

Good job @tsuyoshiwada!
Thanks for contributing!

[wadackel]

@georapbox Thanks you for merging 😄