rapid-cdn: c->h: Really avoid Content-Type modification

Caddy was automat(g)ically adding Content-Type header to the backend
response, but Haproxy does not do it, and change of the servers shall
follow generic approach that not additional information is added to the
response by the CDN.

software/rapid-cdn/test/test.py

@@ -2603,10 +2603,7 @@ class TestSlave(SlaveHttpFrontendTestCase, TestDataMixin, AtsMixin):
       }
     )
 
-    self.assertEqual(
-      'text/xml; charset=utf-8',
-      result.headers['Content-Type']
-    )
+    self.assertNotIn('Content-Type', result.headers)
 
   def test_type_zope_path(self):
     parameter_dict = self.assertSlaveBase('type-zope-path')