Fix discriminator type field update for custom discriminator model name

[wassil]

Summary

Mongoose requires unique model names, so to avoid conflicts when using discriminators, you use 3 parameter syntax: model.discriminator(modelName, schema, discriminatorKeyValue).
When updating a document using e.g. updateOne, you're allowed to change discriminator key value when you provide overwriteDiscriminatorKey.
But the code responsible for updating the discriminator type was relying on modelName being equal to discriminatorKeyValue, which may not be the case if you use custom model name for your discriminator.

Schema.discriminators are indexed by model name and the code was using update value for discriminatorKey.

Examples

I modified an existing test to make it fail, then fixed it.

I'm not sure if I should keep the original test as is and add a new one, but the difference is just 2 lines - the model names for discriminators.

[vkarpov15]

Great, thanks 👍