Atom bidi attribute, empty string value

[James M Snell <jasnell@xxxxxxxxx>]

In an offline note, Franklin Tse asked about the empty string value for
the bidi attribute.  He had raised an issue before about whether that
could be dropped, which I had originally decided to go ahead and do.
However, after going through a number of aggregated feed scenarios,
having the ability to explicitly state that no default direction had
been specified seemed to be more important than relying on a default
direction of LTR.  I can definitely be convinced otherwise, of course.

- James