Copy entity in Graph#fetch

This shouldn't be necessary, but someone is modifying them
in place and it's causing problems elsewhere.

test/spec/graph/graph.js

@@ -59,7 +59,7 @@ describe('Graph', function() {
             var node  = iD.Node({id: "n1"}),
                 way   = iD.Way({id: "w1", nodes: ["n1"]}),
                 graph = iD.Graph({n1: node, w1: way});
-            expect(graph.fetch("w1").nodes).to.eql([node]);
+            expect(graph.fetch("w1").nodes[0].id).to.equal("n1");
         });
     });