Copy entity in Graph#fetch

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

js/id/graph/graph.js

@@ -89,7 +89,7 @@ iD.Graph.prototype = {
     // Resolve the id references in a way, replacing them with actual objects.
     fetch: function(id) {
         var entity = this.entities[id], nodes = [];
-        if (!entity.nodes || !entity.nodes.length) return entity;
+        if (!entity.nodes || !entity.nodes.length) return iD.Entity(entity); // TODO: shouldn't be necessary
         for (var i = 0, l = entity.nodes.length; i < l; i++) {
             nodes[i] = this.fetch(entity.nodes[i]);
         }

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");
         });
     });