Using defaultsDeep, undefined properties get filtered out when they're in the root level of the object, but not when they're nested. If defaultsDeep is meant to act recursively, shouldn't its rules be the same regardless of the nesting level?

To reproduce this issue:

> _.defaultsDeep({}, { foo: undefined })
{}

> _.defaultsDeep({}, { bar: { foo: undefined } })
{ bar: { foo: undefined } }  // by the same rules, shouldn't it be `{ bar: {} }` ?