Redundant "type" property of subclass in class hierarchy

[aldan95]

When created specification for polymorph class model structure there is unnecessary "type: object" property preceding "allOf" in all subclasses (Text and Video in following example):

    "schemas": {
      "Message": {
        "required": [
          "type"
        ],
        "type": "object",
        "properties": {
          "type": {
            "type": "string",
            "enum": [
              "Text",
              "Video"
            ]
          }
        },
        "discriminator": {
          "propertyName": "type",
          "mapping": {
            "Text": "#/components/schemas/Text",
            "Video": "#/components/schemas/Video"
          }
        }
      },
      "Text": {
        "required": [
          "text",
          "type"
        ],
        "type": "object",    # redundant
        "allOf": [
          {
            "$ref": "#/components/schemas/Message"
          },
          {
            "type": "object",
            "properties": {
              "text": {
                "type": "string"
              }
            }
          }
        ]
      },
      "Video": {
        "required": [
          "previewUrl",
          "type",
          "videoUrl"
        ],
        "type": "object",     # redundant
        "allOf": [
          {
            "$ref": "#/components/schemas/Message"
          },
          {
            "type": "object",
            "properties": {
              "videoUrl": {
                "type": "string"
              },
              "previewUrl": {
                "type": "string"
              },
              "text": {
                "type": "string"
              }
            }
          }
        ]
      }
    }

OpenApi spec contains no such property for subclasses.

This extra property leads to errors on api client generation with SwagGen tool.

Sample test controller used:

package test.org.springdoc.api.app5

import com.fasterxml.jackson.annotation.JsonSubTypes
import com.fasterxml.jackson.annotation.JsonTypeInfo
import io.swagger.v3.oas.annotations.media.DiscriminatorMapping
import io.swagger.v3.oas.annotations.media.Schema
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController

enum class MessageType {
	Text, Video
}
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "type")
@JsonSubTypes(
	JsonSubTypes.Type(value = Text::class, name = "Text")
	,JsonSubTypes.Type(value = Video::class, name = "Video")
)
@Schema(
	discriminatorMapping = [
		DiscriminatorMapping(value = "Text", schema = Text::class),
		DiscriminatorMapping(value = "Video", schema = Video::class)
	]
)
abstract class Message(val type: MessageType)

data class Text(
	val text: String
): Message(MessageType.Text)

data class Video(
	val videoUrl: String,
	val previewUrl: String,
	val text: String?
): Message(MessageType.Video)

@RestController
@RequestMapping("/test")
class HelloController {
	@GetMapping
	fun index(): Message? = null
}

moved from springdoc/springdoc-openapi#452