MDL-59784 core: Check for non-promises rather than string

If a malformed value was passed, it may be that we can still display it.
We should not assume that, if a string is not passed, that we are
dealing with a promise.

lib/amd/src/modal.js

@@ -608,7 +608,7 @@ define(['jquery', 'core/templates', 'core/notification', 'core/key_codes',
      */
     Modal.prototype.asyncSet = function(value, setFunction) {
         var p = value;
-        if (typeof value === 'string') {
+        if (typeof value !== 'object' || !value.hasOwnProperty('then')) {
             p = $.Deferred();
             p.resolve(value);
         }