Content-type fix (#659)

* charset fixes

* make linter happy (#661)

---------

Co-authored-by: grulex <god@vk.com>
Co-authored-by: Alexander Baranov <677093+sashabaranov@users.noreply.github.com>

client.go

@@ -107,13 +107,13 @@ func (c *Client) newRequest(ctx context.Context, method, url string, setters ...
 }
 
 func (c *Client) sendRequest(req *http.Request, v Response) error {
-	req.Header.Set("Accept", "application/json; charset=utf-8")
+	req.Header.Set("Accept", "application/json")
 
 	// Check whether Content-Type is already set, Upload Files API requires
 	// Content-Type == multipart/form-data
 	contentType := req.Header.Get("Content-Type")
 	if contentType == "" {
-		req.Header.Set("Content-Type", "application/json; charset=utf-8")
+		req.Header.Set("Content-Type", "application/json")
 	}
 
 	res, err := c.config.HTTPClient.Do(req)

speech.go

@@ -76,7 +76,7 @@ func (c *Client) CreateSpeech(ctx context.Context, request CreateSpeechRequest)
 	}
 	req, err := c.newRequest(ctx, http.MethodPost, c.fullURL("/audio/speech", string(request.Model)),
 		withBody(request),
-		withContentType("application/json; charset=utf-8"),
+		withContentType("application/json"),
 	)
 	if err != nil {
 		return