The delete function failes to delete properties of value undefined

Version: "dot-prop-immutable": "^2.1.1"

Steps to reproduce:

Run the following expression:
```JS
dotProp.delete({a:5, b:undefined, c:undefined}, 'b')
```
Return Value:
```JS
{a: 5, b: undefined, c: undefined}
```
Expected Return Value:
```JS
{a: 5, c: undefined}
```