pass use_json_field to StreamField initializer

Author: pjstevns

wagtailstreamforms/streamfield.py

@@ -44,5 +44,5 @@ def dependencies(self):
 
 class FormFieldsStreamField(StreamField):
     def __init__(self, block_types, use_json_field=None, **kwargs):
-        super().__init__(block_types, **kwargs)
+        super().__init__(block_types, use_json_field=use_json_field, **kwargs)
         self.stream_block = FormFieldStreamBlock(block_types, required=not self.blank)