Fix enum information loss in groovydoc. Fixes #6257

http://jira.codehaus.org/browse/GROOVY-6257
http://groovy.329449.n5.nabble.com/groovydoc-information-loss-for-enums-and-generics-td5716165.html
Merge pull request #243 from mbezjak/enum-information-loss

Fix enum information loss in groovydoc. Fixes #6257