Validating enums is silently skipped if "type" is missing

[tscheburaschka]

If a schema element contains no "type" field, the validation of an "enum" field is silently skipped.
The enum should be validated without type or an error should be raised complaining about the invalid schema.
Example:
schema.json: { "enum": [ "boo", "far"] }
payload.json: "Hello World"
passes silently with obj = 'Hello World'.