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>

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