Allow value_class to be overridden at the field level

wagtail/fields.py

@@ -88,7 +88,7 @@ class StreamField(models.Field):
 
         # extract kwargs that are to be passed on to the block, not handled by super
         block_opts = {}
-        for arg in ["min_num", "max_num", "block_counts", "collapsed"]:
+        for arg in ["min_num", "max_num", "block_counts", "collapsed", "value_class"]:
             if arg in kwargs:
                 block_opts[arg] = kwargs.pop(arg)