Add no-arg overload of selection.data()

There is a no-arg version of [the `data()` method on selections](https://github.com/mbostock/d3/wiki/Selections#wiki-data) which returns the previously-assigned data. From the docs:

> If values is not specified, then this method returns the array of data for the first group in the selection.
Merge pull request #1273 from pelotom/patch-2

Add no-arg overload of selection.data()