[Draft] Bugfix for content_type discriminator issue

[IvanVas]

This isn't the full fix yet, I'd like to get a guidance for the next way to proceed.

Bug:
The discriminatorName in oneOf template is set to contentType if the content_type is used as a discriminator in the OpenAPI spec.
It will then fail in runtime during response deserialization with the error TypeError: typeMap[type].getAttributeTypeMap is not a function.
Same might happen for serialization, didn't test that yet.

We need to make sure "content_type" is used, or that the "content_type" is transformed to "contentType" (might not be desirable).

What are the best next steps?

@amakhrov
@davidgamero
@mkusaka
@joscha

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