Email plugin: dev mailbox errors without trailing slash

[michaelbromley]

Describe the bug
When running the EmailPlugin with the dev mailbox, the URL http://localhost:3000/mailbox gives a blank list with the following errors in the console:

[run:server] error 6/1/20, 10:05 - [Vendure Server] Cannot GET /list
[run:server] error 6/1/20, 10:05 - [Vendure Server] Cannot GET /types

However, http://localhost:3000/mailbox/ (note trailing slash) works as expected.

Expected behavior
Should work with or without trailing slash.

Environment (please complete the following information):

• @vendure/core version: v0.12.5
• Nodejs version: 12.4.0
• Database (mysql/postgres etc): any