Json Builder feature

a small improvement to the json builder (https://jira.codehaus.org/browse/GROOVY-6159) that simplifies the handling of list of objects.

    def songs = [new Song (title: "Title 1"), new Song (title: "Title 2"), ...]

    JsonBuilder json = new JsonBuilder ()
    def map = json {
        songs songs.collect { Song s ->
            json {
                title s.title
            }
        }
    } 

can be written as:

    JsonBuilder json = new JsonBuilder ()
    def map = json {
        songs songs, { Song s ->
            title s.title
        }
    }