[FIX] Make `classed(function)` conform to the interface

This commit makes the `classed(function)` feature conform to the regular `classed` interface.

Instead of:

    .classed(function(d,i) { return d.status })

you need to pass it `true` as second argument:

    .classed(function(d,i) { return d.status }, true)

Note, that get class feature works as expected when the function is single argument:

    .classed(function(d,i) { return d.status })

One test, "gets an existing class by function returning index", is currently still failing,
since I don't know how to access the index for current context/node.