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Adds a new request header (`key`) if not present, otherwise replaces the
previous value of that header with `value`.

It is recommended for header keys to be in lower-case, to avoid sending
duplicate keys in a request. As a convenience, this is validated during
testing which raises a `Plug.Conn.InvalidHeaderError` if the header key
is not lowercase.

Raises a `Plug.Conn.AlreadySentError` if the connection has already been
`:sent` or `:chunked`.
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