[typescript-fetch]Fix subtype serialization with discriminators

Created by: SimonErm

Currently the generated typescript-fetch client only handles the decriminator value on deserialization to resolve the correct subtype, but not for serialization. This PR fixes this by adding the same behaviour for serialization

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Created by: macjohnny

A general question: why is discriminator handling needed for serialization? Can you give an example input and the actual vs expected output?

restored source branch github/fork/SimonErm/fix/discriminator-serialization