Change ApiModelReader to build superset of all visible properties from deserialize/serialize results

This pull request fixes an issue with ApiModelReader that causes the "models" output to exclude any properties that are visible on serialize only. 

Because serialize/deserialize introspections can result in different property lists, you cannot overwrite the result of the serialize introspection with the result of the deserialize introspection without the risk of 'losing' properties. 

The solution is to add new properties to the existing model entry if it exists after serialization.  This effectively creates a superset of all visible serializable and deserializable properties.