text: Title: When Serving static files: "TypeError: 'NoneType' object is not iterable"
  Body: Using the example code:
  ```import responder
  
  api = responder.API()
  
  @api.route("/{greeting}")
  async def greet_world(req, resp, *, greeting):
      resp.text = f"{greeting}, world!"
  
  if __name__ == '__main__':
      api.run()
  ```
  
  I dropped a couple files in the `static/` directory, and while it does successfully serve the files, I get these messages back in the log whenever it loads my index.html file.
  ```
  INFO: ('127.0.0.1', 46560) - "GET /index.html HTTP/1.1" 302
  INFO: ('127.0.0.1', 46560) - "GET / HTTP/1.1" 304
  ERROR: Exception in ASGI application
  Traceback (most recent call last):
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/uvicorn/protocols/http/httptools_impl.py", line 387, in run_asgi
      result = await asgi(self.receive, self.send)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/uvicorn/middleware/message_logger.py", line 59, in __call__
      await self.inner(self.receive, self.send)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/asgiref/wsgi.py", line 41, in __call__
      await self.run_wsgi_app(message)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/asgiref/sync.py", line 108, in __call__
      return await asyncio.wait_for(future, timeout=None)
    File "/usr/lib64/python3.6/asyncio/tasks.py", line 339, in wait_for
      return (yield from fut)
    File "/usr/lib64/python3.6/concurrent/futures/thread.py", line 56, in run
      result = self.fn(*self.args, **self.kwargs)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/asgiref/sync.py", line 123, in thread_handler
      return self.func(*args, **kwargs)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/asgiref/wsgi.py", line 118, in run_wsgi_app
      for output in self.wsgi_application(environ, self.start_response):
  TypeError: 'NoneType' object is not iterable
  
  INFO: ('127.0.0.1', 46564) - "GET /static/chat.js HTTP/1.1" 500
  ERROR: Exception in ASGI application
  Traceback (most recent call last):
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/uvicorn/protocols/http/httptools_impl.py", line 387, in run_asgi
      result = await asgi(self.receive, self.send)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/uvicorn/middleware/message_logger.py", line 59, in __call__
      await self.inner(self.receive, self.send)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/asgiref/wsgi.py", line 41, in __call__
      await self.run_wsgi_app(message)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/asgiref/sync.py", line 108, in __call__
      return await asyncio.wait_for(future, timeout=None)
    File "/usr/lib64/python3.6/asyncio/tasks.py", line 339, in wait_for
      return (yield from fut)
    File "/usr/lib64/python3.6/concurrent/futures/thread.py", line 56, in run
      result = self.fn(*self.args, **self.kwargs)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/asgiref/sync.py", line 123, in thread_handler
      return self.func(*args, **kwargs)
    File "/home/cig/PycharmProjects/PythonSandbox/venv/lib64/python3.6/site-packages/asgiref/wsgi.py", line 118, in run_wsgi_app
      for output in self.wsgi_application(environ, self.start_response):
  TypeError: 'NoneType' object is not iterable
  
  INFO: ('127.0.0.1', 46568) - "GET /static/chat.js HTTP/1.1" 500
  ```
  
  To reproduce:
  1. Check out my [sandbox](https://github.com/chrisbrake/PythonSandbox)
  2. Install `responder`
  3. Go into the `toDo` module
  4. Run `app.py`
  5. Go to http://127.0.0.1:5042/static/