force response to text if not json

Without that I'm getting Version "[object Response]" on the smart card
from an api that returns text/plain.

See #847 - add Docuseal (requires text/plain support or hack)

src/mixins/service.js

@@ -56,7 +56,7 @@ export default {
           throw new Error(`Ping: target not available (${response.status} error)`);
         }
 
-        return json ? response.json() : response;
+        return json ? response.json() : response.text();
       });
     },
   },