DUBBO-396 Spring配置时，filter属性配置上default值时出异常

git-svn-id: http://code.alibabatech.com/svn/dubbo/trunk@1883 1a56cb94-b969-4eaa-88fa-be21384802f2

dubbo-config/dubbo-config-api/src/main/java/com/alibaba/dubbo/config/AbstractConfig.java

@@ -366,6 +366,9 @@ public abstract class AbstractConfig implements Serializable {
                 if (v.startsWith(Constants.REMOVE_VALUE_PREFIX)) {
                     v = v.substring(1);
                 }
+                if (Constants.DEFAULT_KEY.equals(v)) {
+                	continue;
+                }
                 if (! ExtensionLoader.getExtensionLoader(type).hasExtension(v)) {
                     throw new IllegalStateException("No such extension " + v + " for " + property + "/" + type.getName());
                 }