Fix gzip HTTP tests: use -D - to avoid binary body, add owner cert for RDF/XML

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: namedgraph

http-tests/misc/gzip-rdfxml.sh

@@ -9,9 +9,10 @@ purge_cache "$FRONTEND_VARNISH_SERVICE"
 
 # Test that nginx gzip compression is active for RDF/XML dynamic content
 
-response=$(curl -i -k -s \
+response=$(curl -k -s -D - -o /dev/null \
   -H "Accept-Encoding: gzip" \
   -H "Accept: application/rdf+xml" \
+  --cert "$OWNER_CERT_FILE:$OWNER_CERT_PWD" \
   "$END_USER_BASE_URL")
 
 if ! echo "$response" | grep -qi "Content-Encoding: gzip"; then

http-tests/misc/gzip-sefjson.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 
 # Test that nginx gzip compression is active for static JSON (SEF file)
 
-response=$(curl -i -k -s \
+response=$(curl -k -s -D - -o /dev/null \
   -H "Accept-Encoding: gzip" \
   "${END_USER_BASE_URL}static/com/atomgraph/linkeddatahub/xsl/client.xsl.sef.json")