Request-id

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- Fix a bug in ``exclude_prefixes`` where it only matches the first request
on Python 3.

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- Fix changelog.

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- Do not crash when the source header is missing. Instead set the request id
to "-".

- Properly format the UTC offset in the ``{time}`` timestamp.

- Add 100% test coverage.

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- Fix ``logging_level`` option on Python 3.
See https://github.com/mmerickel/request-id/pull/2

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- Catch exceptions and return ``webob.exc.HTTPInternalServerError`` so
that a ``request_id`` may be attached to the response.

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- Fix a couple bugs with ``exclude_prefixes`` and add some docs for it.