io.swagger.parser.util.SwaggerDeserializer#SCHEMA_KEYS shoud contain value "externalDocs"

io.swagger.parser.util.SwaggerDeserializer#SCHEMA_KEYS shoud contain value "externalDocs", because swagger 2 specification allows the use of "externalDocs" in schema objects https://swagger.io/specification/v2/#schemaObject

This error is generated:

> There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
   | Error count: 1, Warning count: 0
  Errors: 
      -attribute definitions.Error.externalDocs is unexpected

for this valid swagger 2 excerption

definitions:
  Error:
    title: Error
    type: object
    description: Default error message template
    required:
      - id
      - code
      - timestamp
      - severity
    externalDocs:
      url: https://example.net/example
      description: Confluence page describing Error model