Update for ModelType ("Stringy" version) change.

CHANGELOG.rst

@@ -5,6 +5,10 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+Studio, LMS: Make ModelTypes more strict about their expected content (for
+instance, Boolean, Integer, String), but also allow them to hold either the
+typed value, or a String that can be converted to their typed value. For example,
+an Integer can contain 3 or '3'. This changed an update to the xblock library.
 
 LMS: Some errors handling Non-ASCII data in XML courses have been fixed.