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>

ipaserver/plugins/dogtag.py

@@ -1832,7 +1832,8 @@ class ra(rabase.rabase, RestClient):
             method='POST',
             headers={'Accept-Encoding': 'gzip, deflate',
                      'User-Agent': 'IPA',
-                     'Content-Type': 'application/xml'},
+                     'Content-Type': 'application/xml',
+                     'Accept': 'application/xml'},
             body=payload
         )