fix(logging): correct invalid logic that determines whether to log a warning or debug message.

Correction for 8d94aed commit.

Author: php-coder

src/main/java/ru/mystamps/web/support/mailgun/ApiMailgunEmailSendingStrategy.java

@@ -131,7 +131,7 @@ public void send(MailgunEmail email) {
 				String.class
 			);
 
-			boolean isWarning = response.getStatusCodeValue() == HttpStatus.OK.value();
+			boolean isWarning = response.getStatusCodeValue() != HttpStatus.OK.value();
 			String body = StringUtils.remove(response.getBody(), '\n');
 			logWarningOrDebug(isWarning, "Mailgun response code: {}", response.getStatusCode());
 			logWarningOrDebug(isWarning, "Mailgun response headers: {}", response.getHeaders());