fix decoding of discriminators when type: object is present

[albertored]

This simply change the order of evalutaion during decode phase checking first if a discriminator is present and then decoding it as an object.

Without this fix the new test would have returned:

%OpenApiSpex.Schema%{
  discriminator: %{
    "propertyName" => "userType"
  },
  oneOf: [
    %{"$ref" => "#/components/schemas/User"},
    %{"$ref" => "#/components/schemas/SpecialUser"}
  ],
  properties: %{},
  type: :object
}

instead of

%OpenApiSpex.Schema%{
  discriminator: %OpenApiSpex.Discriminator{
    extensions: nil,
    mapping: nil,
    propertyName: "userType"
  },
  oneOf: [
    %OpenApiSpex.Reference{"$ref": "#/components/schemas/User"},
    %OpenApiSpex.Reference{"$ref": "#/components/schemas/SpecialUser"}
  ],
  type: "object"
}