Parsing an empty string causes odd behavior

[dpedu]

This code panics on the marked line. It seems like IsValid() incorrectly returns true when it should return false since there are no more tokens available.

p := tokenizer.New()

stream := p.ParseString("")
defer stream.Close()

for stream.IsValid() {
    token := stream.CurrentToken()
    if token.Is(tokenizer.TokenInteger) { // <--- panic
        fmt.Println(token.ValueInt())
    }
    stream.GoNext()
}

I'm using 1.3.0 and the panic stack trace tells me the problem is here:

github.com/bzick/tokenizer.(*Token).Is(...)
	/xxx/go/pkg/mod/github.com/bzick/tokenizer@v1.3.0/token.go:220

[bzick]

I'll fix that

[bzick]

Fixed in 1.4.4