The delete function failes to delete properties of value undefined

[meena-erian]

Version: "dot-prop-immutable": "^2.1.1"

Steps to reproduce:

Run the following expression:

dotProp.delete({a:5, b:undefined, c:undefined}, 'b')

Return Value:

{a: 5, b: undefined, c: undefined}

Expected Return Value:

{a: 5, c: undefined}

[meena-erian]

I have created a pull request #53