Fixed #11400 -- Passed kwargs from AbstractUser.email_user() to send_mail()

Thanks Jug_ for suggestion, john_scott for the initial patch,
and Tim Graham for code review.

docs/releases/1.7.txt

@@ -152,6 +152,10 @@ Minor features
   Each radio button or checkbox includes an ``id_for_label`` attribute to
   output the element's ID.
 
+* Any ``**kwargs`` passed to
+  :meth:`~django.contrib.auth.models.User.email_user()` are passed to the
+  underlying :meth:`~django.core.mail.send_mail()` call.
+
 Backwards incompatible changes in 1.7
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