Add compatibility shim for setAttribute[NS] in IE9.

Similar to style.setProperty, IE9 does not always properly coerce
attribute values to strings.  This causes issues when attempting to set
an instance of d3.rgb as an attribute value.  Now that d3.interpolate
can return d3.rgb instances, this situation can occur more frequently.

This compatiblity shim overrides Element.prototype.setAttribute[NS] so
that all attribute values are coerced to strings in IE9.