Update HalJsonRenderer.groovy

Fixes 3 problems:  
- Java Enums were written out with extra quotes around the "\"ACTIVE\""
- Java dates written out with extra quotes, and also as a date toString() -- some way to force ISO8601?
- GSON "Dangling name" issue due to trying to skip objects that were already written.
- Bug fix: duplicate elements causing GSON errors (due to starting object, then continuing).  Keep track if we've written object.
- Bug fix: Enum and Date types written with quotes and bad formatting.
- Feature: elideDuplicates flag allows control over skipping elements or not
- Feature: dateToStringConverter allows writing dates in custom format
- Add tests.
Merge pull request #417 from jmacnish/master

GRAILS-10672 : fix enums, dates, and repeated elements in HAL JSON rendering