Fixed cover fetching by FAUST number.

For some reason, we missed this bug caused by the changes in PR #48.
One place using ting_get_additional_info() was upgraded to the new
identifier/library-code syntax, but not the other. This PR fixes that,
addressing [LH#1938](https://libraryding.lighthouseapp.com/projects/27862/tickets/1938-soap-error-encoding-object-hasnt-librarycode-property).