[JavaScript] Generated code does not support setting 'Accept' even though claims to

Description

Unable to set accept header

Swagger-codegen version

3.0.46

Swagger declaration file content or url

https://digital.nhs.uk/restapi/oas/357490

Command line used for generation

java -jar /tmp/swagger-codegen-cli-3.0.46.jar generate -i /tmp/357490.json -l javascript -o mesh-client -DuseES6=true -DusePromises=true

Steps to reproduce

Download the OAS file

Run the above command

Attempt to use the client, setting accept header, as in

        const api = new mesh.OutboxApi( client );
	res = await api.sendMessageWithHttpInfo(nums,MAILBOX_ID, auth, nhs_mail_box, 'SPINE_NTT_UPHOLDING', {
		mexFilename:`${local_id}.dat`,
		mexLocalid: local_id,
		accept:'application/vnd.mesh.v2+json'
	});

This will send a request with an accept header of application/json instead of the intended

This is because the sendMessageXxxx method does let accepts = ['application/vnd.mesh.v2+json', 'application/json']; before calling callApi()

In there, at https://github.com/swagger-api/swagger-codegen/blame/master/modules/swagger-codegen/src/main/resources/Javascript/es6/ApiClient.mustache#L440, the existing accept header from headerParams (exposed outside the generated code as opts) is replaced.

Suggest a fix/enhancement

Probably it should only happen if not already set so if (accept) { should be if ( !request.header.accept && accept) {

Start with modules/swagger-codegen/src/main/resources/Javascript/es6/ApiClient.mustache around line 440, then regenerate the JavaScript client from the linked OAS using the command in the issue. Reproduce the request with an opts accept header and verify that the generated client preserves that value instead of replacing it with application/json.