javax.script.Invocable may contain either
• two `invoke' methods
or
• `invokeMethod' and `invokeFunction'
so let's support any variant

Change-Id: I47196af79cc2de75725ded29992ab1f6f4cd623e
Reviewed-on: https://gerrit.libreoffice.org/23163Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>