@Override
    public String toString() {
        return StringFactory.stepString(this, this.format, this.traversalRing);
    }
Reference implementation for String format step, a mid-traversal step which will handle result formatting
to string values. If the incoming traverser is a non-String value then an {@code IllegalArgumentException}
will be thrown.

@author Valentyn Kahamlyk
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/FormatStep.java