Key name from a variable/method

It may be useful to pass keyname for json from somewhere else (look at example below).

```ruby
common_collections_map.each do |key, collection|
  json.key! key, collection  do |item|
    json.call item, :name
  end
end
```

common_collections - hash, mapping some name with some common collections (i.e. different types of tags) which should be included in json.