Merge pull request #1785 from MITx/fix/cdodge/xml_attribute-as-system-metadata

list xml_attributes (new field since xblock integration) as system metad...

common/lib/xmodule/xmodule/x_module.py

@@ -340,7 +340,7 @@ class XModuleDescriptor(XModuleFields, HTMLSnippet, ResourceTemplates, XBlock):
 
     # cdodge: this is a list of metadata names which are 'system' metadata
     # and should not be edited by an end-user
-    system_metadata_fields = ['data_dir', 'published_date', 'published_by', 'is_draft']
+    system_metadata_fields = ['data_dir', 'published_date', 'published_by', 'is_draft', 'xml_attributes']
 
     # A list of descriptor attributes that must be equal for the descriptors to
     # be equal