Add JSONResponse

[sloria]

closes #592

Let me know if any further changes need to be made.

[asvetlov]

I suggest

class JSONResponse(Response):

    def __init__(self, data, *, body=None, status=200,
                 reason=None, headers=None, content_type='application/json',
                 dumps=json.dumps):
        super().__init__(text=text, status=status, reason=reason,
                         content_type=content_type)

[asvetlov]

My former comment was wrong.
See updated proposal:

class JSONResponse(Response):

    def __init__(self, data=sentinel, *, text=None, body=None, status=200,
                 reason=None, headers=None, content_type='application/json',
                 dumps=json.dumps):
        if data is not sentinel and (text of body):
            raise ValueError('only the one of data, text or body should be specified')
        text = dumps(data)
        super().__init__(text=text, body=body, status=status, reason=reason,
                         content_type=content_type)

[sloria]

Changes made. Let me know if further changes are necessary or if I should squash the commits.

[sloria]

What should happen if none of data, text, or body are passed?

The current behavior is not ideal:

JSONResponse()  # TypeError: <object object at 0x10062e400> is not JSON serializable

I can raise an error:

        if not any([data is not sentinel, text, body]):
            raise ValueError(
                'one of data, text, or body must be specified'
            )

but I'm not sure if we want to allow setting any of those attributes after a JSONResponse is instantiated, in which case I can do the following instead:

        if data is not sentinel:
            text = dumps(data)
        super()...

[asvetlov]

I've merged but replaced JSONResponse class with json_response function as @popravich suggested.

[lock]

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