koajs#72 Try to find failing test case, unsuccessfully for now

test/errors.js

@@ -0,0 +1,42 @@
+'use strict'
+
+var request = require('supertest')
+var mount = require('..')
+var Koa = require('koa')
+require('should')
+
+describe('when throwing custom error in mounted app', function () {
+  it('should keep same class', function (done) {
+    var a = new Koa()
+
+    a.use(async function (ctx, next) {
+      throw new CustomError(400)
+    })
+
+    var app = new Koa()
+    app.use(async function (ctx, next) {
+      try {
+        await next()
+        ctx.status = 200
+      } catch (e) {
+        ctx.body = e instanceof CustomError
+        ctx.status = e.status
+      }
+    })
+    app.use(mount(a))
+
+    request(app.listen())
+      .get('/')
+      .expect(400)
+      .expect('true')
+      .end(done)
+  })
+})
+
+class CustomError extends Error {
+  constructor (status, message) {
+    super(message)
+    this.status = status
+    Error.captureStackTrace(this, CustomError)
+  }
+}