In the following pipeline, I would expect that appending the variable `y` to `jsonArray` would be the same regardless if it is a Java String or a GString. Right now, I'm must call `.toString()` before appending.

 
```
pipeline {
  agent any

  stages {
    stage('Test') {
      steps {
        script {
          def x = "x"
          def y = "${x}"
          def jsonArray = readJSON text: '[]'
          jsonArray << x
          jsonArray << y
          println(jsonArray)
        }
      }
    }
  }
}
```

Actual output:

```
["x",

{"valueCount":1,"strings":["",""],"bytes":[120],"values":["x"]}

]
```

Expected output:

```
["x","x"]
```