AtomFormatter should not require "author" on every entry.

[jlunman]

The atom spec says:

atom:entry elements MUST contain one or more atom:author elements,
unless ... the atom:feed element contains an atom:author element itself.

So if there's a feed-level author, a per-entry author is not required. Requiring it makes feed documents needlessly verbose.

Ideally, the following lines should just be removed. The AtomFormatter should probably just format the data as given, and not try to enforce rules where it lacks sufficient state to do so.

SyndicationFeedReaderWriter/src/Atom/AtomFormatter.cs

Lines 316 to 319 in 149fa2e

	if (!hasAuthor)
	{
	throw new ArgumentException("Author is required");
	}