Bug 31393: Koha::Config: handle the special case for 'content' attribute

Bug 31393: Koha::Config: handle the special case for 'content' attribute

<key content="value"></key>

was wrongly parsed as

{ key => 'value' }

whereas it should be

{ key => { content => 'value' } }

The 'content' attribute is used in shibboleth config

Test plan:
1 Run `prove t/Koha/Config.t`

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>