Fixed Plugin.tags not initialising

If the constructor is passed an object that doesn't have a `tags` key.

src/gui/html/js/plugin.js

@@ -181,7 +181,7 @@
       this.priority = obj.priority || 0;
       this.isPriorityGlobal = obj.isPriorityGlobal || false;
       this._messages = obj.messages || [];
-      this._tags = obj.tags;
+      this._tags = obj.tags || [];
       this._isDirty = obj.isDirty || false;
 
       /* UI state variables */

src/tests/gui/html/js/test_plugin.js

@@ -183,6 +183,24 @@ describe('Plugin', () => {
       plugin.isPriorityGlobal.should.be.true();
     });
 
+    it('should set tags value to an empty array if no key was passed', () => {
+      const plugin = new loot.Plugin({ name: 'test' });
+
+      plugin.tags.length.should.equal(0);
+    });
+
+    it('should set tags to passed key\'s value', () => {
+      const tags = [{
+        name: 'Delev',
+      }];
+      const plugin = new loot.Plugin({
+        name: 'test',
+        tags,
+      });
+
+      plugin.tags.should.deepEqual(tags);
+    });
+
     it('should set id to the plugins name without spaces', () => {
       const plugin = new loot.Plugin({ name: 'test plugin name' });