Previously, a LinkDescriptor had to be created with both a rel and a
description. If a description was not provided a failure would occur.
This commit relaxes the above-described restriction by allowing a
link's title to be used as its default description. If a descriptor
has a description, it will always be used irrespective of whether or
not the link has a title. If the descriptor does not have a
description and the link does have a title, the link's title will be
used. If the descriptor does not have a description and the link does
not have a title a failure will occur.
Closes gh-105
c4b7438708