converse-disco: don't replace a pending promise

I'm not sure how much this is an issue outside of tests, where we might
run into race conditions arising to the fact that we don't always
respond to every IQ stanza

src/headless/converse-disco.js

@@ -566,7 +566,7 @@ converse.plugins.add('converse-disco', {
                  * @namespace _converse.api.disco.features
                  * @memberOf _converse.api.disco
                  */
-                'features': {
+                features: {
                     /**
                      * Return a given feature of a disco entity
                      *
@@ -642,7 +642,9 @@ converse.plugins.add('converse-disco', {
                     entity.features.reset();
                     entity.fields.reset();
                     entity.identities.reset();
+                    if (!entity.waitUntilFeaturesDiscovered.isPending) {
                         entity.waitUntilFeaturesDiscovered = utils.getResolveablePromise()
+                    }
                     entity.queryInfo();
                     return entity.waitUntilFeaturesDiscovered;
                 },