Update z_datacitedoi.pl to set datecitedoi repoid to securehost if host is undefined

[drn05r]

Recent version of EPrints has introdiced a simplified means for configuration EPrints for HTTPS only:

https://wiki.eprints.org/w/Simplified_HTTPS_Configuration

This means that there could be issue with setting $c->{datacitedoi}->{repoid} if HTTPS only configuration is defined in the archive's cfg/cfg.d/10_core.pl.

DataCiteDoi/cfg/cfg.d/z_datacitedoi.pl

Line 27 in 86ac3a7

$c->{datacitedoi}{repoid} = $c->{host};

If this line is changed to something like:

c->{datacitedoi}{repoid} = defined $c->{host} ? $c->{host} : $c->{securehost};

This should ensure DataCiteDOI still works with HTTPS-only configuration for EPrints 3.4.1+.