Merge pull request #4118 from jribbens/master

Fix convert_to_valid_json migrations

src/oscar/apps/basket/migrations/0010_convert_to_valid_json.py

             at.value = json.dumps(val, cls=DjangoJSONEncoder)
             at.save()
             continue
-        except ValueError:
+        except (ValueError, SyntaxError):
             pass
 
         # convert the string to json as it is

src/oscar/apps/order/migrations/0012_convert_to_valid_json.py

             at.value = json.dumps(val, cls=DjangoJSONEncoder)
             at.save()
             continue
-        except ValueError:
+        except (ValueError, SyntaxError):
             pass
 
         # convert the string to json as it is