ZF-8102: Relative URIs in Atom links may fail to be constructed from the base URI value of the feed
When parsing an Atom link element, the presence of an empty href attribute fails to trigger a URI construction using the base URI of the feed. Essentially an empty href attribute should be interpreted as being equal to the base URI of the feed.
This can be fixed by detecting relative URIs more robustly and holding the base URI of the feed as a separate cached value to simplify implementation. Some checking should be done to ascertain whether this also effects the RSS parsing class and perhaps add a getBaseUri() method to the feed and entry APIs.