Remove trailing slash in proxyUrl

Infura has trouble handling requests with a trailing slash in API endpoint. INFURA/infura#221

Credit to 🍯 for the find

src/server.mjs

@@ -46,6 +46,7 @@ function getReqBody(req) {
 
 async function proxy(host, request, reqBody, response) {
   let pathname = url.parse(request.url).pathname;
+  pathname = pathname === "/" ? "" : pathname; // to avoid a trailing slash
   let hostname = url.parse(host).host;
   let proxyUrl = `${host}${pathname}`;
   let reqMethod = request.method;