Only flush once startGzip has been called

This fixes #58 and prevents the underlying Flusher
from writing the wrong status code or writing
headers before Content-Encoding has been set.

This is tmthrgd/gziphandler@cb0f3d94c6 with the test
case taken from tmthrgd/gziphandler@574da8f22d.

Author: tmthrgd

gzip.go

@@ -192,10 +192,16 @@ func (w *GzipResponseWriter) Close() error {
 // http.ResponseWriter if it is an http.Flusher. This makes GzipResponseWriter
 // an http.Flusher.
 func (w *GzipResponseWriter) Flush() {
-	if w.gw != nil {
-		w.gw.Flush()
+	if w.gw == nil {
+		// Only flush once startGzip has been called.
+		//
+		// Flush is thus a no-op until the written body
+		// exceeds minSize.
+		return
 	}
 
+	w.gw.Flush()
+
 	if fw, ok := w.ResponseWriter.(http.Flusher); ok {
 		fw.Flush()
 	}

gzip_test.go

@@ -306,6 +306,24 @@ func TestStatusCodes(t *testing.T) {
 	}
 }
 
+func TestFlushBeforeWrite(t *testing.T) {
+	b := []byte(testBody)
+	handler := GzipHandler(http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		rw.WriteHeader(http.StatusNotFound)
+		rw.(http.Flusher).Flush()
+		rw.Write(b)
+	}))
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	r.Header.Set("Accept-Encoding", "gzip")
+	w := httptest.NewRecorder()
+	handler.ServeHTTP(w, r)
+
+	res := w.Result()
+	assert.Equal(t, http.StatusNotFound, res.StatusCode)
+	assert.Equal(t, "gzip", res.Header.Get("Content-Encoding"))
+	assert.NotEqual(t, b, w.Body.Bytes())
+}
+
 func TestIgnoreSubsequentWriteHeader(t *testing.T) {
 	handler := GzipHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(500)