Enable string type schemas for enums.

This code allows the generation of schemas for enums rendered as strings. I modified the existing GetSchema method in JsonConverter, adding a type parameter in order to enumerate the available enum names. This change to the interface of JsonConverter does not appear to cause any problems, as GetSchema was only called in one place: the JsonSchemaGenerator class.

This fixes the GenerateSchemaWithEnumString test, and so the Ignore attribute on that test has been removed.