Spaces should be allowed in brands

[pzduniak]

... or user should not be allowed to generate unparseable messages.

    smsg, err := saltpack.SignArmor62(
        saltpack.CurrentVersion(),
        []byte("message here"),
        secretKey,
        "TWO PART",
    )
    if err != nil {
        panic(err)
    }

    skey, msg, brand, err := saltpack.Dearmor62Verify(
        saltpack.CheckKnownMajorVersion,
        smsg,
        kr,
    )
    if err != nil {
        panic(err) // will panic here, unable to parse the header
    }