In IBM Api connect Unable to Change the response Content-Type to “application/json; charset=UTF-8”

Question

I am using apic.setvariable('message.headers.Content-Type', 'application/json; charset=UTF-8'); property in gateway script to change response header. Which is not working.

Suspecting, the content-type is not charset=utf8, which in turn overwriting the content-type header. How to use Map Policy to transform the response body to charset=utf-8?

Any help would be appreciated!