datetime incompatibility with JSON schema and OpenAPI specification

[nbrede]

Currently datetimes are supported by specifying the type as "datetime" in a schema. This seems to be incompatible with the JSON schema and OpenAPI specifications, which state it should be called "date-time". See, e.g., https://json-schema.org/understanding-json-schema/reference/string.html#dates-and-times

I created a PR to fix that issue: #91