Update mockttp to get client errors & drop default headers

Author: pimterry

package-lock.json

@@ -48,7 +48,7 @@
     "graphql": "^14.0.2",
     "graphql-yoga": "^1.18.1",
     "lodash": "^4.17.13",
-    "mockttp": "^0.19.3",
+    "mockttp": "^0.20.0",
     "node-fetch": "^2.6.0",
     "node-forge": "^0.9.0",
     "node-gsettings-wrapper": "^0.5.0",