Trying to figure out a Grails behaviour

TestController.groovy

//This happens with the default 2.3.3 Config.groovy:
//curl 'http://localhost:8080/test-accept-header/test/index' -H 'Accept: application/json, */*'
//FIRST BLOCK
//curl 'http://localhost:8080/test-accept-header/test/index' -H 'Accept: application/json'
//WILDCARD BLOCK

class TestController {
	def index() {
		withFormat {
			xml {
				render text:'FIRST BLOCK'
			}
			'*' {
				render text: 'WILDCARD BLOCK'
			}
		}
	}
}