UNKNOWN value on unset User-Agent header

[elpiel]

When parsing a request without an User-Agent, woothee returns UNKNOWN (string), however for a cleaner API I would suggest to have some sort of an enum or just an Option if the parser couldn't determine the given part.

This way a user of the crate (me) wouldn't need to filter the UNKNOWN string in order to get an Option that will show whether there is a value parsed.

Update: I saw that the rest of the woothee libraries do the same and I understand this might be to have consistency across them, however I still want to see your thoughts about this.