[httpserver.py] self.get_arguments do NOT work for 'DELETE' and 'OPTIONS' http method

There may be some disagreement on the understanding of HTTP Protocol between Tornado and jQuery.
I found sometimes I can not use self.get_argument() to get the data. So I conduct a test:

```CoffeeScript
for method in ['GET', 'POST', 'HEAD', 'PUT', 'PATCH', 'OPTIONS', 'DELETE']
  $.ajax
    url: '/api/test'
    type: method
    data:
      foo: 'bar'
    success: (evt) ->
        console.log 'foobar'
```

I send each of ''GET', 'POST', 'HEAD', 'PUT', 'PATCH', 'OPTIONS', 'DELETE'' method in $.ajax, examine the HTTP request with Chrome debugger and the results of self.get_argument in RequestHandler, finding that

| method  | GET | POST | HEAD | PUT | PATCH | OPTIONS | DELETE
| ------------- | ------------- | ------------- | ------------- | ------------- | ------------- | ------------- | ------------- |
| in Query String paras  | Yes | no | Yes | no | no | no | no |
| in Form data |  no | Yes | no | Yes | Yes | Yes | Yes |
| in self.request.arguments | Yes | Yes | Yes | Yes | Yes | no | no |
| in self.request.body | no | Yes | no | Yes | Yes | Yes | Yes |
| in self.get_argument | Yes | Yes | Yes | Yes | Yes | no | no |

When the method is OPTIONS or DELETE, I can not use self.get_argument('foo') to get the value I set in ajax request.

Since I'm not familiar with HTTP protocol, I do not understanding the line of code.
Is is a bug? I guess.