plainToClass - discriminator is deleted in plain/source object

[hohler]

When using plainToClass with a plain object in combination with @type and a discriminator, it will delete the discriminator from that plain object.
A workaround is either to clone the object, or to have keepDiscriminatorProperty set to true on the @type definition. But then it will also be kept in the target object.

I think the root cause is in TransformOperationExecutor.ts on line 51:
if (!targetType.options.keepDiscriminatorProperty) delete subValue[targetType.options.discriminator.property];

I can't make a fix / PR right now because of lack of time, but I like to have the issue documented here.

[alexpls]

@hohler I could replicate this by adding the following test: b402d62

I'd be happy to work on a PR to fix this once the project is under active maintenance again.