fix for inheriting metadata bug

* problem was on import to json--got all the metadata,
  but didn't preserve the _inherited_metadata
* added own_metadata property, use it instead

common/lib/xmodule/xmodule/modulestore/xml.py

@@ -41,6 +41,9 @@ class ImportSystem(XMLParsingSystem, MakoDescriptorSystem):
         self.used_slugs = set()
 
         def process_xml(xml):
+            """Takes an xml string, and returns a XModuleDescriptor created from
+            that xml.
+            """
             try:
                 # VS[compat]
                 # TODO (cpennington): Remove this once all fall 2012 courses

common/lib/xmodule/xmodule/modulestore/xml_importer.py

@@ -35,6 +35,8 @@ def import_from_xml(store, data_dir, course_dirs=None, eager=True,
             store.update_item(module.location, module.definition['data'])
         if 'children' in module.definition:
             store.update_children(module.location, module.definition['children'])
-        store.update_metadata(module.location, dict(module.metadata))
+        # NOTE: It's important to use own_metadata here to avoid writing
+        # inherited metadata everywhere.
+        store.update_metadata(module.location, dict(module.own_metadata))
 
     return module_store

common/lib/xmodule/xmodule/x_module.py

@@ -358,6 +358,14 @@ class XModuleDescriptor(Plugin, HTMLSnippet):
         self._child_instances = None
         self._inherited_metadata = set()
 
+    @property
+    def own_metadata(self):
+        """
+        Return the metadata that is not inherited, but was defined on this module.
+        """
+        return dict((k,v) for k,v in self.metadata.items()
+                    if k not in self._inherited_metadata)
+
     def inherit_metadata(self, metadata):
         """
         Updates this module with metadata inherited from a containing module.