JSON requests support

[kasun]

This is a minor enhancement when dealing with JSON requests.

Right now to do a json request we have to do something like this,

headers = {
    'Content-Type': 'application/json'
}
yield Request(url, method='POST', body=json.loads(data), headers=headers)

I'm proposing a new Request subclass that can save a few keystrokes.

yield JSONRequest(url, data=data)

Will be sending a PR for this.

[kmike]

Fixed by #3505 .