Fix prefixIds plugin to properly handle url()s in style="..." (#1592)

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

```javascript
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.

plugins/prefixIds.js

@@ -206,15 +206,16 @@ exports.fn = (_root, params, info) => {
             node.attributes[name] != null &&
             node.attributes[name].length !== 0
           ) {
-            // extract id reference from url(...) value
-            const matches = /url\((.*?)\)/gi.exec(node.attributes[name]);
-            if (matches != null) {
-              const value = matches[1];
-              const prefixed = prefixReference(prefix, value);
-              if (prefixed != null) {
-                node.attributes[name] = `url(${prefixed})`;
+            node.attributes[name] = node.attributes[name].replace(
+              /url\((.*?)\)/gi,
+              (match, url) => {
+                const prefixed = prefixReference(prefix, url);
+                if (prefixed == null) {
+                  return match;
+                }
+                return `url(${prefixed})`;
               }
-            }
+            );
           }
         }