Just one data point for you:  good-old 7-bit ASCII is a proper subset of the UTF-8 encoding of Unicode.  In other words, an ASCII strings is a UTF-8 string.  You can save the last byte of your message header, since it will always have the same value.

Just one data point for you:  good-old 7-bit ASCII is a proper subset of the UTF-8 encoding of Unicode.  In other words, an ACII strings is a UTF-8 string.  You can save the last byte of your message header, since it will always have the same value.