can't set header, need more_set_headers or something

[themotu]

I only want to add cors header if one doesn't exist, more_set_headers seems to do this but extras aren't installed?

I'm running a lot of instances of the same site on different domains. One issue I have is that I want swagger to connect from swagger1234.mydomain.com to api1234.mydomain.com swagger compains that cors aren't passed.

add_header in a conf.d breaks my app because it adds a second cors header but makes swagger work on the one endpoint. adding an api1234.mydomain.com in vhost.d doesn't add the header.