Fix value of enum to be Array

[takahashim]

I am trying to use grape-swagger and other OpenAPI tools, but currently there seems to be a problem with the schema generated by grape-swagger.

In the OpenAPI 2.0 specification, enum uses the JSON Schema enum.

• https://github.com/OAI/OpenAPI-Specification/blob/f6f9ab3d88ad6ff0b235d5f4dc1a8ab28941d895/schemas/v2.0/schema.json#L1001-L1003
• https://swagger.io/specification/v2/

In JSON Schema, the value of an enum is expected to be an Array.

• https://datatracker.ietf.org/doc/html/draft-fge-json-schema-validation-00#section-5.5.1

However, in grape-swagger, an enum value can be a String literal.

This pull request fixes that and returns an Array.

I hope this change can be accepted.

[LeFnord]

thanks @takahashim … looks reasonable for me, please add an entry to the UPGRADE

[takahashim]

Thanks a lot! Also added a line to UPGRADING.md.