Old style streaming responses no longer supported in Django 1.7

[frewsxcv]

https://github.com/educreations/django-piston/blob/master/piston/resource.py#L246

django/django@8b9b8d3#diff-eeebc8cb56240caf13a9952431189fef

Blocking us from upgrading to Django 1.7

[streeter]

Ugh, sad face.

[frewsxcv]

I think we should be able to replace:

    @staticmethod
    def _use_emitter(result):
        """True iff result is a HttpResponse and contains non-string content."""
        if not isinstance(result, HttpResponse):
            return False
        elif django.VERSION >= (1, 4):
            return result._base_content_is_iter
        else:
            return not result._is_string

with

    @staticmethod
    def _use_emitter(result):
        """True iff result is a HttpResponse and contains non-string content."""
        return isinstance(result, HttpResponse)

Since:

1. We don't care about django.VERSION < (1, 4)
2. Since HttpStreamingResponse is not a subclass of HttpResponse (it's a subclass of HttpResponseBase), we should always expect HttpResponse to contain string content (I think)

[frewsxcv]

I take back what I wrote previously in the code block. Since HttpResponse should always return string content, that method should always return False

    @staticmethod
    def _use_emitter(result):
        """True iff result is a HttpResponse and contains non-string content."""
        return False