Feature/save custom attributes

When I was running this code it was executing the else statement, which tries to exectue to execute the `class_attr.__class__.__name__` method on `self`. Usually `class_attr.__class__.__name__` is `InstrumentedAttribute` or `ColumnProperty` or `DateTime`, but in our case it's `instancemethod` - since custom_attributes is just a simple method. I wasn't able to get it to work with decorators such as `ColumnProperty` and I didn't feel comfortable adding `instancemethod` method to `UpdateAttrHandler` so I just added an elif statement that executes only if the else part would fail (so that this changes as little as possible).