Enforce type of the XBlocks Fields when set

This updates the XBlock dependency to edX/XBlock@***
and add the newly introduced enforce_type methods

common/lib/xmodule/xmodule/fields.py

@@ -68,10 +68,7 @@ class Date(Field):
         """
         if value is None:
             return None
-        if isinstance(value, time.struct_time):
-            # struct_times are always utc
-            return time.strftime('%Y-%m-%dT%H:%M:%SZ', value)
-        elif isinstance(value, datetime.datetime):
+        if isinstance(value, datetime.datetime):
             if value.tzinfo is None or value.utcoffset().total_seconds() == 0:
                 # isoformat adds +00:00 rather than Z
                 return value.strftime('%Y-%m-%dT%H:%M:%SZ')
@@ -80,6 +77,8 @@ class Date(Field):
         else:
             raise TypeError("Cannot convert {!r} to json".format(value))
 
+    enforce_type = from_json
+
 TIMEDELTA_REGEX = re.compile(r'^((?P<days>\d+?) day(?:s?))?(\s)?((?P<hours>\d+?) hour(?:s?))?(\s)?((?P<minutes>\d+?) minute(?:s)?)?(\s)?((?P<seconds>\d+?) second(?:s)?)?$')
 
 
@@ -117,6 +116,15 @@ class Timedelta(Field):
                 values.append("%d %s" % (cur_value, attr))
         return ' '.join(values)
 
+    def enforce_type(self, value):
+        """
+        Ensure that when set explicitly the Field is set to a timedelta
+        """
+        if isinstance(value, datetime.timedelta) or value is None:
+            return value
+
+        return self.from_json(value)
+
 
 class RelativeTime(Field):
     """
@@ -219,3 +227,12 @@ class RelativeTime(Field):
         if len(stringified) == 7:
             stringified = '0' + stringified
         return stringified
+
+    def enforce_type(self, value):
+        """
+        Ensure that when set explicitly the Field is set to a timedelta
+        """
+        if isinstance(value, datetime.timedelta) or value is None:
+            return value
+
+        return self.from_json(value)

requirements/edx/github.txt

@@ -17,7 +17,7 @@
 -e git+https://github.com/appliedsec/pygeoip.git@95e69341cebf5a6a9fbf7c4f5439d458898bdc3b#egg=pygeoip
 
 # Our libraries:
--e git+https://github.com/edx/XBlock.git@aed7a2c51a59836e435259ad0fb41f8e865fa530#egg=XBlock
+-e git+https://github.com/FiloSottile/XBlock.git@a9158e9887217abe27e451d1ff82c0d9f98c0bd6#egg=XBlock  # TODO: put here the edx/XBlock#200 merge commit
 -e git+https://github.com/edx/codejail.git@71f5c5616e2a73ae8cecd1ff2362774a773d3665#egg=codejail
 -e git+https://github.com/edx/diff-cover.git@v0.5.0#egg=diff_cover
 -e git+https://github.com/edx/js-test-tool.git@v0.1.5#egg=js_test_tool