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>
(cherry picked from commit 6973625e7a06e66ee731ea8f5b69f296943c580f)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 35ae7b467e54c38b8f99e392b02838e9fd53e9e0)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>