RSS2 Google PubSubHubbub problem

[juanforlizzi]

Describe the bug
Only one atom:link in RSS2 is posible at the moment.
If you have atomLink and hub option at the same time, only the last one is added to the feed.

To Reproduce
Create a feed using this options :

feed: 'https://example.com/feed/rss.xml',
hub: 'https://pubsubhubbub.appspot.com/',

Expected behavior

<atom:link href="https://example.com/feed/rss.xml" rel="self" type="application/rss+xml"/>
<atom:link href="https://pubsubhubbub.appspot.com/" rel="hub"/>

Actual behavior

<atom:link href="https://pubsubhubbub.appspot.com/" rel="hub"/>

[juanforlizzi]

@jpmonette I've opened #184 to fix the issue.
Please take a look.