Make Dogtag return XML for ipa cert-find
    
    Using JSON by default within Dogtag appears to cause ipa cert-find to
    return JSON, when the request was made with XML. We can request that XML
    is returned as before by specifying so in the request header.
    
    Fixes: https://pagure.io/freeipa/issue/8980
    Signed-off-by: Chris Kelley <ckelley@redhat.com>
    Reviewed-By: Francois Cami <fcami@redhat.com>