Describe the bug
Direct Transfer fails when source site has multiple 'link' http headers. The specific situation occured when trying to transfer a site from wp-engine.

To Reproduce
Steps to reproduce the behavior:

1. initiate a direct transfer with a site that has multiple 'link' http headers.
2. Fails with error Uncaught TypeError: explode(): Argument #2 ($string) must be of type string, array given in /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/admin/migrate/class-boldgrid-backup-admin-migrate-rx-rest.php:376

Expected behavior
Expected the plugin to extract the json link from the link header string.