pull request for adding content type to grouper client

Hello! I'm a student from the University of Hawaii working on a project that utilizes Grouper and GrouperClient. We are in the process of updating our codebase from Java 8 to Java 17 and have run into this unique situation where our Grouper is still on version 2.2 while our project needs to use a version of Grouper Client that supports Java 17. The main issue is that the POST requests made by GrouperClientWs sets the default content type of the request header to application/json, which is unrecognized by the Grouper server, as it only accepts text/x-json. So this PR adds a method to assign the content type to all Gc classes that use the GrouperClientWs class to make the POST request. The only Gc class left unchanged is GcLdapSearchAttribute since it doesn't use the GrouperClientWs class.