Fix several issues in view feed of com_tags component

#PR summary:

This PR fixes issue #4623 reported by @javigomez and several coding errors which I found while reviewing code in view feed of com_tags component:

1. The getListQuery method of TagsModelTags component doesn't return "author" and "author_email", so there are notice errors in the following lines of code in view feed:

https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L56
https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L74

2. $item->displayDate in the line of code https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L57 need to be replaced with $item->created_time (there is no field displayDate in the $item object)

3. $item in the following lines of code need to be replaced by $feeditem

https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L70
https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L74

4. $row->author_email needs to be replaced by $item->author_email (I think it is typing mistake)

https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L74

#Testing instructions:
For some reasons, these notice errors are not displayed when I tried to view the feed, so it will be difficult for end-users to test this pull request (these errors are logged in error logs of the web server). Maybe one of PLTs (or developers who understand Joomla code) can help reviewing this code and get it merged? @mbabker Could you help review this code? The errors are clearly mentioned above. 

For end-users, the only think you can test is:

- View rss feed for tags component. You will see that for each feed item, the author and pubDate tag are not available for each feed item.
- Apply this pull request. Check it again and you will see that author and pubDate are available now for each feed item.