[BUG][KOTLIN][Retrofit2] Generator should support optional header parameter

Created by: alitams

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Header parameters can be optional, like path parameters. See the code below. What I experience, despite of the required attribute is false, it generates the parameter as required. I expect that it should be optional with null default value.

Experienced:

@PUT("verify")
fun verify(@Body verifyRequest: VerifyRequest, @Header("Token") verifyData: String): Single<VerifyResponse>

Expected:

@PUT("verify")
fun verify(@Body verifyRequest: VerifyRequest, @Header("Token") verifyData: String? = null): Single<VerifyResponse>

openapi-generator version

5.0.1

OpenAPI declaration file content or url

   /verify:
    put:
      tags:
        - verify
      operationId: verifyData
      parameters:
        - $ref: '#/components/parameters/optionalToken'
      requestBody:
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/VerifyRequest"
        required: true
      responses:
        "200":
          description: Successfully verifed
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/VerifyResponse"

components:
  parameters:
    optionalToken:
      name: Token
      in: header
      required: false
      description: OptionalToken description
      schema:
        type: string

Suggest a fix

The kotlin-client/libraries/jvm-retrofit2/headerParams.mustache should be changed from this:

{{#isHeaderParam}}@Header("{{baseName}}") {{{paramName}}}: {{{dataType}}}{{/isHeaderParam}}

to this:

{{#isHeaderParam}}@Header("{{baseName}}") {{paramName}}: {{{dataType}}}{{#required}}{{#defaultValue}} = {{{.}}}{{/defaultValue}}{{/required}}{{^required}}?{{#defaultValue}} = {{{.}}}{{/defaultValue}}{{^defaultValue}} = null{{/defaultValue}}{{/required}}{{/isHeaderParam}}