transition.{attr,style} should use interpolateNumber if the target value is a number.

[mbostock]

#67 fixed the short-circuit comparison when transition.attr or transition.style is passed a constant, but changed the interpolator to use interpolateString rather than interpolateNumber. (And I don’t recall my reasoning in #67 (comment), since the interpolator is chosen based on the target value, not the current value.) We should still use interpolateNumber if the target value is a number.