Sprockets.mixins.mediatype

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- Pin :mod:`ietfparse` to avoid breakages introduced in 1.5.0.

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- Add :func:`sprockets.mixins.mediatype.content.install`.
- Add :func:`sprockets.mixins.mediatype.content.get_settings`.
- Deprecate :meth:`sprockets.mixins.mediatype.content.ContentSettings.from_application`.
- Update to ietfparse 1.4.

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- Set the :http:header:`Vary` header if we are setting the content type.

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- Removed deprecation wrapper since it seems to cause really interesting
problems including the much feared meta-class error.

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- Repackage from a module into a package. Distributing raw modules inside
of a namespace package is unreliable and questionably correct.
- Add :func:`sprockets.mixins.mediatype.content.add_transcoder`.
- Add :class:`sprockets.mixins.mediatype.transcoders.JSONTranscoder`.
- Add :class:`sprockets.mixins.mediatype.transcoders.MsgPackTranscoder`.
- Add :class:`sprockets.mixins.mediatype.transcoders.BinaryWrapper`.
- Normalize registered MIME types.
- Raise a 400 status when content body decoding fails.

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- Support using the default_content_type in the settings if request does not
contain the Accept header