fix: ignore errors when forcefully closing the socket (#601)

In order to catch the following errors:

```
events.js:288
      throw er; // Unhandled 'error' event
      ^

Error: write EPIPE
    at afterWriteDispatched (internal/stream_base_commons.js:154:25)
    at writeGeneric (internal/stream_base_commons.js:145:3)
    at Socket._writeGeneric (net.js:780:11)
    at Socket._write (net.js:792:8)
    at doWrite (_stream_writable.js:441:12)
    at writeOrBuffer (_stream_writable.js:425:5)
    at Socket.Writable.write (_stream_writable.js:316:11)
    at abortConnection (<myproject>/node_modules/engine.io/lib/server.js:506:12)
    at <myproject>/node_modules/engine.io/lib/server.js:353:7
    at Server.verify (<myproject>/node_modules/engine.io/lib/server.js:158:14)
    at Server.handleUpgrade (<myproject>/node_modules/engine.io/lib/server.js:351:8)
```

Closes #596, #598

lib/server.js

@@ -503,6 +503,9 @@ function sendErrorMessage(req, res, code) {
  */
 
 function abortConnection(socket, code) {
+  socket.on("error", () => {
+    debug("ignoring error from closed connection");
+  });
   if (socket.writable) {
     const message = Server.errorMessages.hasOwnProperty(code)
       ? Server.errorMessages[code]

test/server.js

@@ -802,6 +802,27 @@ describe("server", function() {
       });
     });
 
+    it("should abort connection when upgrade fails", done => {
+      listen({ allowUpgrades: true }, port => {
+        const req = http.request(
+          {
+            port,
+            path: "/engine.io/",
+            headers: {
+              connection: "Upgrade",
+              upgrade: "websocket"
+            }
+          },
+          res => {
+            expect(res.statusCode).to.eql(400);
+            res.resume();
+            res.on("end", done);
+          }
+        );
+        req.end();
+      });
+    });
+
     it(
       "should trigger if a poll request is ongoing and the underlying " +
         "socket closes, as in a browser tab close",