[MRI] Silence dupe-id error on setting attribute.

Fixes sparklemotion#1262.

ext/nokogiri/xml_node.c

@@ -778,34 +778,44 @@ static VALUE namespaced_key_eh(VALUE self, VALUE attribute, VALUE namespace)
  *
  * Set the +property+ to +value+
  */
-static VALUE set(VALUE self, VALUE property, VALUE value)
+static VALUE set(VALUE node_rb, VALUE property_name_rb, VALUE property_value_rb)
 {
   xmlNodePtr node, cur;
-  xmlAttrPtr prop;
-  Data_Get_Struct(self, xmlNode, node);
+  xmlChar*   property_name ;
+  xmlAttrPtr property;
+
+  Data_Get_Struct(node_rb, xmlNode, node);
+
+  if (node->type != XML_ELEMENT_NODE) {
+    return(Qnil); // TODO: would raising an exception be more appropriate?
+  }
+
+  property_name = (xmlChar *)StringValuePtr(property_name_rb);
 
   /* If a matching attribute node already exists, then xmlSetProp will destroy
    * the existing node's children. However, if Nokogiri has a node object
    * pointing to one of those children, we are left with a broken reference.
    *
    * We can avoid this by unlinking these nodes first.
    */
-  if (node->type != XML_ELEMENT_NODE)
-    return(Qnil);
-  prop = xmlHasProp(node, (xmlChar *)StringValuePtr(property));
-  if (prop && prop->children) {
-    for (cur = prop->children; cur; cur = cur->next) {
+  property = xmlHasProp(node, property_name);
+  if (property && property->children) {
+    for (cur = property->children; cur; cur = cur->next) {
       if (cur->_private) {
         nokogiri_root_node(cur);
         xmlUnlinkNode(cur);
       }
     }
   }
 
-  xmlSetProp(node, (xmlChar *)StringValuePtr(property),
-      (xmlChar *)StringValuePtr(value));
+  xmlResetLastError();
+  xmlSetStructuredErrorFunc(NULL, Nokogiri_error_silencer);
+
+  xmlSetProp(node, property_name, (xmlChar *)StringValuePtr(property_value_rb));
+
+  xmlSetStructuredErrorFunc(NULL, NULL);
 
-  return value;
+  return property_value_rb;
 }
 
 /*