[JavaScript] Generated code doesn't set default value for header params per swagger spec

Description

When a default value is specified for a header parameter (e.g. a Link header), the generated JavaScript code allows the user to specify a value (correct), but does not provide a default value if an explicit value is not given (seemingly incorrect).

Swagger-codegen version

2.4.x (tried on 2.4.0, 2.4.2, 2.4.8, 2.4.9)

Swagger declaration file content or url

https://github.com/LD4P/sinopia_server/blob/e3283b3e87e020a3f29793f4076ddb8c73cf46d5/swagger.yaml#L68-L73

Command line used for generation

java -jar swagger-codegen-cli-2.4.8.jar generate -i swagger.yaml -l javascript --additional-properties usePromises=true -o sinopia_client/

Steps to reproduce

run the aforementioned command on the above linked swagger spec.

get generated JS code that results in having to manually implement the default, e.g.:
https://github.com/LD4P/sinopia_server/blob/e3283b3e87e020a3f29793f4076ddb8c73cf46d5/sinopia_client/src/api/LDPApi.js#L84

Related issues/PRs

Suggest a fix/enhancement

it would be great if the generated code would allow the user to specify header param values as it does, while also providing a default value if the swagger spec lists one and the user provides no explicit value.

Start with the linked swagger.yaml header parameter definition and the generated sinopia_client/src/api/LDPApi.js example, then reproduce generation with the documented swagger-codegen-cli command. Trace the JavaScript generator output for header parameters and verify that an omitted explicit value uses the Swagger default while still allowing overrides.