- 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.

Conflicts:
	grails-plugin-rest/src/main/groovy/grails/rest/render/hal/HalJsonRenderer.groovy