LP#1306823: try to increase changes of fetching added content from the OL

LP#1306823: try to increase changes of fetching added content from the OL

For works that have more than one edition in the Open Library's
database, using 'isbn' as the search key can sometimes fail to
bring up the edition that has a cover image.  This patch instead
uses "isbn_10" or "isbn_13", depending on the length of the ISBN,
to try to retrieve the right edition.

This is known to work with ISBN 9264023607.  However, it is also
entirely possible that this works only because of a quirk in OL
as of the date of this patch; for example, neither "isbn_10"
nor "isbn_13" are listed as valid ID types in
https://openlibrary.org/dev/docs/api/read.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>