When sending a query via POST, the content-type is not set. The content charset for the POST parameters are set, but this only appears to be used for creating the Content-Length header in the commons library. Since a query is encoded in UTF-8, the http headers should also specify content type charset.
On Tomcat, this causes problems when the query string contains non-ascii characters (characters with accents and such) as it tries to parse the POST body in its default ISO-9886-1. There appears to be no way to set/change the default encoding for a message body on Tomcat.