canMakeObserve: strictly allowed types

Made objects for canMakeObserve more strict; only allow plain JS objects
or existing instances of can.Observe or derived classes (such as
can.Observe.List, can.Model, etc. )

Allowing other things to be wrapped into can.Observe instances may cause
problems as well, so it is better to white-list and be conservative
about what is and is not allowed.

(For example; even wrapping can.Control, which is part of CanJS itself
throws.)

observe/observe.js

@@ -6,12 +6,7 @@ steal('can/util','can/construct', function(can, Construct) {
 	//  
 	// Returns `true` if something is an object with properties of its own.
 	var canMakeObserve = function( obj ) {
-			return ( obj
-				&& !obj.nodeType
-				&& !can.isWindow( obj )
-				&& !(obj instanceof can.$ )
-				&& !(obj instanceof Date)
-				&& ( "object" === typeof obj ));
+			return obj && ( can.isPlainObject( obj ) || ( obj instanceof can.Observe ));
 		},
 
 		// Removes all listeners.