Bug 26847: (follow-up) fix get_attribute() call

Bug 26847: (follow-up) fix get_attribute() call

Since the upgrade to Selenium 3 in bug 27055, an additional parameter is
required for get_attribute() to actually work.
https://metacpan.org/pod/Selenium::Remote::WebElement#get_attribute

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit de739259298b323bb881b62b1bc96bc9e3589496)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 04a0e973331c9210ab8324efb9fb7575148081d7)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>