setting one Stateful as the value of another copies `_watchCallbacks`

[ben hockey]

when setting one Stateful as the value for another, the watchers of the Stateful being passed are then also effectively watching the Stateful that is being set. the diff below is for tests/Stateful.js to demonstrate the problem

tests/Stateful.js

@@ -20 +20 @@
                 doh.is(5, s.get("foo"));
         },
         function setHash(t){
-                var s = new dojo.Stateful();
+                var s = new dojo.Stateful(),
+                        fooCount = 0,
+                        handle = s.watch('foo', function () {
+                                fooCount++;
+                        });
                 s.set({
                         foo:3,
                         bar: 5
                 });
                 doh.is(3, s.get("foo"));
                 doh.is(5, s.get("bar"));
+                doh.is(1, fooCount);
                 var s2 = new dojo.Stateful();
                 s2.set(s);
                 doh.is(3, s2.get("foo"));
                 doh.is(5, s2.get("bar"));
+                // s watchers should not be copied to s2
+                doh.is(1, fooCount);
+                handle.unwatch();
         },
         function wildcard(t){
                 var s = new dojo.Stateful();

i found this while looking at #15187 but since this issue has existed all along i opened it as a new ticket.

[bill]

Guess this should be assigned to Kris unless Kitson wants to fix it.

[Kris Zyp]

In [29038]:

Don't copy watch callbacks, fixes #15315 !strict