Bug 32923: Fix x-koha-embed from EHoldings/*/*

Bug 32923: Fix x-koha-embed from EHoldings/*/*

We were validating twice the input and it broke the x-koha-embed header.
I don't really know what's happening, but here is how to recreate the
problem:
1. Edit Koha/REST/V1/ERM/EHoldings/Packages/EBSCO.pm
2. Add the following lines in Koha/REST/V1/ERM/EHoldings/Packages/EBSCO.pm, in the list method
    warn $c->req->headers->header('x-koha-embed');
3. curl -v -s -u koha:koha --request GET http://kohadev-intra.mydnsname.org:8081/api/v1/erm/eholdings/ebsco/packages --header "x-koha-embed: resources+count,vendor"

=> There is no vendor in the response
=> The warn displays "resources+count"

If you apply this patch you will notice that "vendor" is part of the
response and the warn displays "resources+count, vendor"

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>