[Broker-J] Header validation in management-http plugin

The method accept() in RestServlet.java (HTTP management plugin) includes unvalidated data in an HTTP response header. This enables attacks such as cache-poisoning, cross-site scripting, cross-user defacement, page hijacking, cookie manipulation or open redirect.

Same issue affects method setContentDispositionHeaderIfNecessary() in AbstractServlet.java.

Appropriate validation should be added.