Gets a header field on message.
The caller is responsible for checking the type of the returned #GVariant matches what is expected.
A 8-bit unsigned integer (typically a value from the #GDBusMessageHeaderField enumeration)
A #GVariant with the value if the header was found, null otherwise. Do not free, it is owned by message.
Gets a header field on message.
The caller is responsible for checking the type of the returned #GVariant matches what is expected.