Fix prefixIds plugin to properly handle url()s in style="..."

[IlyaSkriblovsky]

First of all, thanks for the great tool!

prefixIds plugin currently breaks url()-links inside style attributes:

optimize(
  `<g style="fill:url(#brush-id);stroke:url(#pen-id)"/>`,
  { plugins: ['prefixIds'] }
).data

will generate <g style=""/>. Seems like prefixIds assumes that attribute's whole value might be url(), but this is not the case for the style attribute.

This fix solves the issue by preserving all attribute's content other than #id inside url(). It also adds some more tests for the prefixIds plugin.

[TrySound]

Thanks!