public String toString() {
        return StringFactory.stepString(this, Arrays.asList(this.propertyKeys),
                this.valueTraversal, this.returnType.name().toLowerCase());
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/PropertyMapStep.java