Configuration of sortable columns

Problem/Motivation

The configuration of sortable columns and the default order in Footable Settings of a View are ignored. All columns can be sorted.

Proposed resolution

It seems that the setting is not properly introduced into HTML attributes data-sortable and data-sorted. I changed the boolean values to strings and then it worked - sorting of columns works for those configured as such. Columns not configured to be sortable, are not sortable any more. See patch attached.

Remaining tasks

Please review the patch, maybe there is a better place to fix this issue.