BranchStep pickToken should be integrated when added as a child option

For pickToken traversal under branch step isRoot() method returns a true result. I think it is because we are not invoking this.integrateChild(pickOptionTraversal) here https://github.com/apache/tinkerpop/blob/master/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/branch/BranchStep.java#L87