Update Mockttp for HTTP/2 & httpolyglot fixes

Quite a few other small changes in here, but nothing that should be
impactful for HTTP Toolkit itself.

Author: pimterry

package-lock.json

@@ -76,7 +76,7 @@
     "mime-types": "^2.1.27",
     "mobx": "^6.3.5",
     "mockrtc": "^0.3.1",
-    "mockttp": "^3.9.4",
+    "mockttp": "^3.10.1",
     "node-fetch": "^2.6.1",
     "node-forge": "^1.3.0",
     "node-gsettings-wrapper": "^0.5.0",