Use repr instead of str for getting context data via the system render function,…

Use repr instead of str for getting context data via the system render function, so it works better for non-ASCII data.

common/lib/xmodule/xmodule/tests/__init__.py

@@ -33,8 +33,8 @@ def test_system():
     """
     Construct a test ModuleSystem instance.
 
-    By default, the render_template() method simply returns the context it is
-    passed as a string.  You can override this behavior by monkey patching::
+    By default, the render_template() method simply returns the repr of the
+    context it is passed.  You can override this behavior by monkey patching::
 
         system = test_system()
         system.render_template = my_render_func
@@ -46,7 +46,7 @@ def test_system():
         ajax_url='courses/course_id/modx/a_location',
         track_function=Mock(),
         get_module=Mock(),
-        render_template=lambda template, context: str(context),
+        render_template=lambda template, context: repr(context),
         replace_urls=lambda html: str(html),
         user=Mock(is_staff=False),
         filestore=Mock(),