fix(core): keep key order when rendering a Map

[yvrng]

The VariableRenderer#render() method currently uses a HashMap to store rendered values, and this Map implementation does not preserve order.

This causes a problem when the in parameter is a LinkedHashMap, and you want the order to be preserved.
For example, when using the Produce task in the Kafka plugin (here) with an Avro serializer: the event value is part of the map that is first rendered; the order of the keys must be preserved in order to respect the schema, and currently this leads to serialization problems.

I changed it to LinkedHashMap, ensuring that key ordering is maintained in case a LinkedHashMap is passed as an input.

[loicmathieu]

LGTM