[kotlin] Fix enum defaultValue in query parameters with schema reference

[daberni]

In kotlin Codegen, when a query parameter is a reference to an enum with a default value specified, the related Api was not generated correctly:

paths:
  /status:
    get:
      operationId: getStatus
      parameters:
        - name: state
          in: query
          schema:
            $ref: '#/components/schemas/PetStatus'
components:
  schemas:
    PetStatus:
      type: string
      enum:
        - available
        - pending
      default: available

fun getStatus(state: PetStatus? = available) : Unit {
    …
}

The value for state must have the enum type prepended, and also the value doesn't respect the additional property enumPropertyNaming: UPPERCASE.

With my fix, both issues get resolved, resulting in a compilable Api:

fun getStatus(state: PetStatus? = PetStatus.AVAILABLE) : Unit {
    …
}

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[wing328]

thanks for the PR

what about adding a docstring explaining what this function does?