fix(autocomplete): improve promise logic

Thx @winniehell.

Closes #6521.

src/components/autocomplete/js/autocompleteController.js

@@ -634,21 +634,29 @@ function MdAutocompleteCtrl ($scope, $element, $mdUtil, $mdConstant, $mdTheming,
    */
   function fetchResults (searchText) {
     var items = $scope.$parent.$eval(itemExpr),
-        term  = searchText.toLowerCase();
-    if (angular.isArray(items)) {
-      handleResults(items);
-    } else if (items) {
+        term  = searchText.toLowerCase(),
+        isList = angular.isArray(items);
+
+    if ( isList ) handleResults(items);
+    else          handleAsyncResults(items);
+
+    function handleAsyncResults(items) {
+      if ( !items ) return;
+
+      items = $q.when(items);
       setLoading(true);
       promiseFetch = true;
+
       $mdUtil.nextTick(function () {
-        if (items.success) items.success(handleResults);
-        if (items.then)    items.then(handleResults);
-        if (items.finally) items.finally(function () {
-          setLoading(false);
-          promiseFetch = false;
-        });
+          items
+            .then(handleResults)
+            .finally(function(){
+              setLoading(false);
+              promiseFetch = false;
+            });
       },true, $scope);
     }
+
     function handleResults (matches) {
       cache[ term ] = matches;
       if ((searchText || '') !== ($scope.searchText || '')) return; //-- just cache the results if old request