Parse HTTP headers in web feed discovery function

[capjamesg]

In draft PR #8, I noted:

I also wonder whether HTTP headers should be included in this function, too. Parsing HTTP headers is not too difficult. I can imagine edge cases where someone may provide a feed as a HTTP header but not as a header.

This is outwith the scope of the PR in which the proposal was made.

The idea is that we should parse parse headers in thediscover_web_page_feeds() function. This will ensure that feeds will be returned for pages who only specify their feeds in a header. While this is an edge case, accounting for this case will make the feed discovery function more robust.

[capjamesg]

This feature has been implemented so I am closing this issue.