Deserialization bug model

[patrickleonardy]

Bug Report

Description

It seems that after deserialization the _is_fitted attribute of the LinearRegressionModel object is set to false, even if the model that was serialized already had it set to True.
Check if this is also the case for the LogisticRegressionModel.

Steps to Reproduce

1. Create a model (LinearRegressionModel)
2. fit the model
3. serialize it
4. deserialize it
5. check the attributes

Expected Result

I was expecting that the _is_fitted attribute is equal to True (as it was before serializing/ deserializing)

Actual Results

I actually got _is_fitted = False