add new step to use summary when there are unrecognized properties

[manton]

In the Post Type Algorithm, in between steps "If the post has a 'photo' property with a valid URL" and "If the post has a 'content' property with a non-empty value", add a new step to use summary as the content if there are any unrecognized properties.

This came out of a discussion with how Micro.blog interprets Micropub requests from IndieBookClub, which sends read-of and read-status with summary but no content. If a Micropub server always prefers user-entered content without this extra check for unrecognized parameters, the new post would lose any extra context about the book that might otherwise have been in summary. Also a problem for bookmark-of.

@tantek, @aaronpk , @gRegorLove: feel free to add any other details from the discussion. Thanks!