GROOVY-5918 Remove metaPropertyValues from Expando before calling toJson

Since https://github.com/groovy/groovy-core/commit/ecfe0f06efc2293fac5310caec9c501f50e597de calling `JsonBuilder` on an `Expando` seems to have thrown a Stack Overflow exception

Fingers crossed this fixes it, by specifically handling `Expando` in the `toJson` overloaded methods (Inside JsonOutput)