get() from undefined should return default value

[Radosław Mejer]

I think that it would be a good case when dotProp.get() returns defualt value even when given object is not an instance of Object.

Quick example:

var fields = undefined;

dotProp.get(fields, 'attributes.required', false);
// false

Right now it returns the passed object.

[Sindre Sorhus]

Agreed. Pull request welcome :)