servant-client: Support adding trailing slashes to API requests

I'm attempting to use servant-client to communicate with an API that expects all routes to end with a trailing slash. On GET requests, they return a redirect status code, so the client ends up at the proper API endpoint eventually. However, on POST/PUT/DELETE, they just return `Method Not Allowed`. It would be great if there was a way to configure servant to output optionally output trailing slashes.

A more general idea might be to introduce a function to ClientM that modifies each request prior to sending or something along those lines.