don't serialize weight out as an int - use a string. The code in capa-module.py expects a string and tries to convert to a float. So this impedence mismatch is causing a crash on import when importing empty strings for 'weight' or 'attempts'

common/lib/xmodule/xmodule/xml_module.py

@@ -128,8 +128,7 @@ class XmlDescriptor(XModuleDescriptor):
         'graded': bool_map,
         'hide_progress_tab': bool_map,
         'allow_anonymous': bool_map,
-        'allow_anonymous_to_peers': bool_map,
-        'weight': int_map
+        'allow_anonymous_to_peers': bool_map
     }