Bug 13758: Correct KOHA::VERSION in OverDrive.pm

Bug 13758: Correct KOHA::VERSION in OverDrive.pm

Correct $KOHA::VERSION to $Koha::VERSION.
Also, passing this string to LWP::UserAgent is wrong. It expects key/value
pairs. Since this string is apparently intended as an agent, this patch
passes it as such.
Note: The OverDrive has unfortunately no unit tests.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested this change in the perl debugger with just these lines:
    use C4::External::OverDrive;
    C4::External::OverDrive::_request();
Printed $ua->agent while stepping into sub _request.
Without the agent change, the adjusted Koha string would just be ignored
and I would still have "libwww-perl/6.04" as agent.
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>