close http.Server after test ended

running a > 100 tests using supertest I had the problem that the socket hang up.

Error: socket hang up
      at createHangUpError (http.js:1442:15)
      at Socket.socketOnEnd [as onend] (http.js:1538:23)
      at Socket.g (events.js:175:14)
      at Socket.EventEmitter.emit (events.js:117:20)
      at _stream_readable.js:910:16
      at process._tickCallback (node.js:415:13)

to prevent this without to increase the kern.maxfiles and kern.maxfilesperproc I have to close the opened http.Server after the test ended.
Test for closing server in .end(fn).
Merge pull request #119 from invernizzie/master

Close server after test has ended.