Writer: fixed header being improperly reset

lz4.go

@@ -101,3 +101,7 @@ type Header struct {
 	CompressionLevel int    // Compression level (higher is better, use 0 for fastest compression).
 	done             bool   // Header processed flag (Read or Write and checked).
 }
+
+func (h *Header) Reset() {
+	h.done = false
+}

writer.go

@@ -384,7 +384,7 @@ func (z *Writer) Reset(w io.Writer) {
 	n := cap(z.c)
 	_ = z.close()
 	z.freeBuffers()
-	z.Header = Header{}
+	z.Header.Reset()
 	z.dst = w
 	z.checksum.Reset()
 	z.idx = 0