Check for Column in by()

The by(Function) doesn't check for Column:

https://github.com/apache/tinkerpop/blob/a14cf28f45742f47f15b768a90355ca9e9e89e17/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/ByModulating.java#L57

as it does here:

https://github.com/apache/tinkerpop/blob/a14cf28f45742f47f15b768a90355ca9e9e89e17/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/ByModulating.java#L94

Seems like it would be better to just add:

if (function instanceof Column)
    this.modulateBy(new ColumnTraversal((Column) function))

Mentioned here on the user list: https://groups.google.com/d/msg/gremlin-users/Y0fez_aLdqs/nyUKQkO_AAAJ