Rename _get_val_from_obj to value_from_object

value_from_object has been in Django for a long time. _get_val_from_obj is deprecated

wagtail/wagtailcore/fields.py

@@ -119,7 +119,7 @@ class StreamField(with_metaclass(models.SubfieldBase, models.Field)):
         return super(StreamField, self).formfield(**defaults)
 
     def value_to_string(self, obj):
-        value = self._get_val_from_obj(obj)
+        value = self.value_from_object(obj)
         return self.get_prep_value(value)
 
     def get_searchable_content(self, value):

wagtail/wagtailsearch/index.py

@@ -123,7 +123,7 @@ class BaseField(object):
     def get_value(self, obj):
         try:
             field = self.get_field(obj.__class__)
-            value = field._get_val_from_obj(obj)
+            value = field.value_from_object(obj)
             if hasattr(field, 'get_searchable_content'):
                 value = field.get_searchable_content(value)
             return value