feat: model_id tag add omitempty (#3)

* Fix compile error from variable typo

* bytes.Buffer pointer

client/tts.go

@@ -83,7 +83,7 @@ func (c Client) TTS(ctx context.Context, text, voiceID, modelID string, options
 		b := bytes.Buffer{}
 
 		defer res.Body.Close()
-		io.Copy(w, res.Body)
+		io.Copy(&b, res.Body)
 		return b.Bytes(), nil
 	case 422:
 		fallthrough

client/types/types.go

@@ -19,7 +19,7 @@ type Voice struct {
 	Labels      string     `json:"labels,omitempty"`      // Serialized labels dictionary for the voice.
 }
 type TTS struct {
-	ModelID       string           `json:"model_id"`
+	ModelID       string           `json:"model_id,omitempty"`
 	Text          string           `json:"text"`                     // The text that will get converted into speech. Currently only English text is supported.
 	VoiceSettings SynthesisOptions `json:"voice_settings,omitempty"` // Voice settings are applied only on the given TTS request.
 }