HtmlUnit is logging those warnings. They are coming to standard out because your project doesn't define / configure a Logger.
Two options to get rid of those warnings if you can not change the content-type response from the server
1) (Easiest) Configure a logging mechanism to ignore warnings from com.gargoylesoftware.htmlunit (log4j, slf4j, LOGBack)
2) To override the logging from going anywhere: Create a class that extends the HtmlUnitDriver to have a public method getWebClient that returns the HtmlUnit WebClient, call the setIncorrectnessListener method with another new class that is an implementation of IncorrectnessListener which just has an empty notify() method.