Cannot detect it is multiline string or object

[nasa9084]

Related to: #75 (fixed by #78)

if the receiver type is interface{} and unmarshaling with UnmarshalYAML([]byte), it seems impossible to detect it is multiline string or object.

e.g. the value in https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v3.0/api-with-examples.yaml#L18 and the value in https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v3.0/api-with-examples.yaml#L51 have same type but the result of unmarshal is not same, they are defined as interface{} and former one should be unmarshaled to map[string]interface{}, later one should unmarshaled to string

before #78, the passed bytes contains | at first character so we could detect it is string or object

is there any way to detect? (or, should revert the change?)