stripUnknown ignores keys with value undefined

[npavlovichero]

Describe the bug
Cast called with stripUnknown doesn't strip keys with value undefined. Not even with .strict(true) set.

To Reproduce

Reproducible example

Expected behavior
Since { key1: 'a', key2: undefined } is not the same as { key1: 'a' }, I expect cast called with { stripUnknown: true } to remove unknown keys regardless of their value. At the very list if strict(true) is set.

Platform (please complete the following information):

• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.