This override returns "utf-8" if the character encoding in the request is null. It works around a Struts
issue (http://issues.apache.org/bugzilla/show_bug.cgi?id=29668), where CommonsMultipartRequestHandler uses
the character encoding from the request, which causes problems if the encoding wasn't specified in the request
(as it usually never is). This is tracked in Beehive's JIRA as
http://issues.apache.org/jira/browse/BEEHIVE-803 .