Don't remove empty patterns with inheritance

plugins/removeEmptyContainers.js

@@ -24,6 +24,7 @@ var container = require('./_collections').elemsGroups.container;
  */
 exports.fn = function(item) {
 
-    return !(item.isElem(container) && !item.isElem('svg') && item.isEmpty());
+    return !(item.isElem(container) && !item.isElem('svg') && item.isEmpty() &&
+        (!item.isElem('pattern') || !item.hasAttr('xlink:href')));
 
 };

test/plugins/removeEmptyContainers.01.svg

@@ -1,4 +1,5 @@
 <svg xmlns="http://www.w3.org/2000/svg">
+    <pattern/>
     <g>
         <marker>
             <a/>

test/plugins/removeEmptyContainers.02.svg

@@ -1,4 +1,10 @@
-<svg xmlns="http://www.w3.org/2000/svg">
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+    <defs>
+        <pattern id="a">
+            <rect/>
+        </pattern>
+        <pattern xlink:href="url(#a)" id="b"/>
+    </defs>
     <g>
         <marker>
             <a/>
@@ -9,7 +15,13 @@
 
 @@@
 
-<svg xmlns="http://www.w3.org/2000/svg">
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+    <defs>
+        <pattern id="a">
+            <rect/>
+        </pattern>
+        <pattern xlink:href="url(#a)" id="b"/>
+    </defs>
     <g>
         <path d="..."/>
     </g>