Sprockets.mixins.http

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- Added an option to control response body transformation for errors, i.e. HTTP
status code >= 400. By default, a JSON or otherwise structured response body
will be reduced down to its error ``message``. That can be overridden by
setting ``simplify_error_response`` to False.
- Fix compile-time setting of default argument values in ``http_fetch``.

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- Remove the lrucache on response body due to a bug in behavior

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- Add handling of ``tornado.httpclient.HTTPTimeoutError`` and
``tornado.httpclient.HTTPStreamClosedError`` exceptions
- Fix documentation builds
- Update documentation links to readthedocs.io

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- Cast the ``url`` parameter of ``http_fetch`` to a string.
Allows for native use of URL abstractions like `yarl <https://yarl.readthedocs.io/en/latest/>`_.

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- Fix a bug with the rejected consumer User-Agent behavior

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- Refactor the HTTPResponse to a stand-alone class
- Add ``history`` attribute of the response with all response objects
- Add ``links`` attribute of the response with the parsed link header if set
- Add ``exceptions`` attribute with stack of exceptions returned as responses
- Add ``dont_retry`` as argument to ``http_fetch`` method
- Change logging level in a few places to a more appropriate level
- Add support for rejected consumers when auto-creating the ``User-Agent`` header
- Add the netloc of a request to the log entry created when rate limited
- Use RequestHandler.settings instead of RequestHandler.application.settings
when auto-creating the ``User-Agent`` header for a Tornado request handler
- Add test coverage of the Warning response header behavior