add tests for SuperAgent request

types/superagent/superagent-tests.ts

@@ -390,3 +390,18 @@ request
     .then(response => {
         // reads 404 page as a successful response
     });
+
+// Test that the "Plugin" type from "use" provides a SuperAgentRequest rather than a Request,
+// which has additional properties.
+const echoPlugin = (request: request.SuperAgentRequest) => {
+  req.url = '' + req.url;
+  req.cookies = '' + req.cookies;
+  if (req.method) {
+    req.url = '/echo'
+  }
+};
+
+request
+    .get('/echo')
+    .use(echoPlugin)
+    .end();