Sprockets.mixins.http

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- Fix failure when response lacks Content-Type header

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- Add logging of response ``Warning`` headers

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- Add support for Python 3.6 and 3.7
- Add support for Tornado < 6

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- Add ``max_redirects`` keyword param
- Add ``validate_cert`` keyword param
- Fix log records always using default number of attempts

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- Change the hard pin requirement on umsgpack

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- Add ``max_http_attempts`` keyword param