Update AbstractFrame.php (#52)

Allow underscore in xpath attributes (required for example in cz-nic (.cz) optional parameter)

src/AfriCC/EPP/AbstractFrame.php

@@ -128,7 +128,7 @@ protected function createNodes($path)
                 $node_name = $matches[1];
             }
             // check if attribute needs to be set
-            elseif (preg_match('/^(.*)\[@([a-z0-9]+)=\'([a-z0-9]+)\'\]$/i', $node_name, $matches)) {
+            elseif (preg_match('/^(.*)\[@([a-z0-9]+)=\'([a-z0-9_]+)\'\]$/i', $node_name, $matches)) {
                 $node_name = $matches[1];
                 $attr_name = $matches[2];
                 $attr_value = $matches[3];