cli.BuildURL: Allow for trailing slash at the end of the path

[babolivier]

Some endpoints (e.g. https://matrix.org/speculator/spec/HEAD/client_server/unstable.html#get-matrix-client-r0-pushrules) require a trailing slash at the end of the path, but cli.BuildURL seems to strip it from the built URL. Possibly tricky as it might come from the url package.