Copy cookie policy when copying a CookieJar

Without it, the cookie policy would get lost while making a request.
That would be invisible to the user if the default cookie policy wasn't
changed, of course.

requests/cookies.py

@@ -415,6 +415,7 @@ class RequestsCookieJar(cookielib.CookieJar, collections.MutableMapping):
     def copy(self):
         """Return a copy of this RequestsCookieJar."""
         new_cj = RequestsCookieJar()
+        new_cj.set_policy(self._policy)
         new_cj.update(self)
         return new_cj