Grails 3 support

[lxshao]

We like this plugin, it was used to bypass our SSO in our dev environment for Grails 2. Now We moved to Grails 3. I took your source and made it working with Grails 3. Are you interested to incorporate my changes?

[knoxilla]

@lxshao I am interested in your updates for Grails 3. Could you make a pull request here, or direct me to your code elsewhere on github? Would be very useful! Thanks much.