Make selected option label match the available sorting options.

[Pineirin]

The sort options are defined in a dictionary. Currently, the list of possible sort options come from the title attribute of the mentioned dictionary, while the option that appears selected in the dropdown item comes from the keys of the dictionary. This may cause mismatch between the selected option on the dropdown item and the one selected on the dropdown menu.

Origin and more information can be found in: inveniosoftware/invenio-administration#132 (comment).