It redirects exactly because of double slash, so I assume it should help. If you are using Linux, here is binary for you based on our upcoming release tyk-fixed-double-slash
It is NOT for production use, but will give you feeling if bug is fixed.
It will be released at the start of next week. Unfortunately, do not think this can be fixed with URL rewrites: this is behaviour of low-level HTTP handling library we use. The only alternative is to compile it manually