Fixing check for "description" request param in JMXJsonServlet.java

When trying to access the JMX metrics' description via the "description=true" URL parameter, any value is accepted.

The current version checks only if "description" is in the URL parameter, but doesn't check the parameter value. 

I would like to fix this via checking if the parameter value is 'true' and showing the description only when this condition is met.