Avoid exception in mixin when target[prop] == null

When target[prop] is null then source[prop] should be assigned directly without calling mixin again. It would cause an exception because null.hasOwnProperty is undefined, of course.
Merge branch 'master' of github.com:requirejs/i18n
Integrate feedback from jrburke to fix exception in mixin method
Now, if target[prop] is not defined we assign a new object to it and then run mixin as usual.
Merge pull request #13 from AIM360/master

Avoid exception in mixin when target[prop] == null