[RSS sitemaps] Extract links from <guid> elements

[sebastian-nagel]

Some RSS feeds use <guid> elements to mark links. At least, if there is no <link> element, URLs should be also extracted from <guid>.

One example item from https://antarcticsun.usap.gov/resources/xml/antsun-science.xml:

<item>
  <title>Why Antarctic Fish Don't Freeze Their Tails Off</title>
  <description>An innovative project to understand how fish survive in the frigid Antarctic waters is opening up new avenues for researchers monitoring what goes on under the sea ice in McMurdo Sound. Evolutionary biologist Paul Cziko from the University of Oregon is studying how Antarctic fish don’t, themselves, freeze into a solid block while spending their lives in subzero waters.</description>
  <enclosure url="https://antarcticsun.usap.gov/science/images7/rss_4349-moocamera-lg.jpg" length="65539" type="image/jpeg" />
  <category>The Biological World</category>
  <pubDate>Tue, 10 Apr 2018 09:38:10 GMT</pubDate>
  <guid isPermaLink="true">https://antarcticsun.usap.gov/science/contentHandler.cfm?id=4349</guid>
  <linktext target="_blank">Read the Story</linktext>
  <altimage>https://antarcticsun.usap.gov/science/images7/rss_4349-moocamera-sm.jpg</altimage>
  <imagecaption>Paul Cziko dives under the frozen ice to install the McMurdo Oceanographic Observatory</imagecaption>
</item>

[sebastian-nagel]

Note: the RSS spec says that "all elements of an item are optional" while the Atom spec requires at least one element as child of <entry>.