JSON converter is not working in unit tests in Grails

If calling grails.converters.JSON from unit test throws error like:

java.lang.RuntimeException: org.codehaus.groovy.grails.web.converters.exceptions.ConverterException: Unconvertable Object of class: my.blog.Post
	at org.codehaus.groovy.grails.web.converters.AbstractConverter.toString(AbstractConverter.java:111)
	at org.codehaus.groovy.runtime.InvokerHelper.format(InvokerHelper.java:616)
	at org.codehaus.groovy.runtime.InvokerHelper.format(InvokerHelper.java:559)

It is easy to fix this error by adding ControllerUnitTestMixin to the test definition like:

@TestMixin(ControllerUnitTestMixin)
@TestFor(BlogService)
class BlogServiceTests {
...