Fix: NBT custom writer

[Edouard127]

This pull request fixes issues with gzip/zlib writers, where the NBT is not written to the buffer until the writer is closed

Example:

func TestEncoder_EncodeNil(t *testing.T) {
    type Struct struct {
        Hi string
    }

    arg := Struct{
        Hi: "A",
    }

    var buf bytes.Buffer
    writer := zlib.NewWriter(&buf)
    err := NewEncoder(writer).Encode(arg, "")
    if err != nil {
        t.Fatal(err)
    }

    writer.Close() // Removing this does not write to the buffer

    fmt.Println(buf.Bytes())
}

[Tnze]

No, it's your responsibility to flush or close the writer.

For example, consider the case we are sending NBT through network, of course we can't close the connection every time we send a NBT message.