Kill raw part when getting mutable reference to typed header

[andresilva]

Fixes #821.

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• Commit: c471ce6
  • Commits must be in the following format: %{type}(%{scope}): %{description}

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[seanmonstar]

Thanks! Just a couple comments.

• I think a typed could exist from parsing by getting typed(), and then later on a call to typed_mut() would not remove the raw representation.
• Could you add a couple tests for the multiple combinations that might cause it to be out of sync?
• Since the commit message becomes part of the changelog, could it be something tamer? :D Maybe just replacing "kill" with "remove"?