Add CORS_ORIGIN environment variable

[arel]

Fixes #18

Previously, ssl-proxy assigned the SERVER_NAME value to the Access-Control-Allow-Origin header (which was not documented).

One issue is that any CORS headers set by the proxied server are overridden here. Another issue is that there is no way to customize the CORS headers, which is important for any API.

In my case, I would have preferred ssl-proxy not override the server's headers, but at the very least I needed to be able to set the Access-Control-Allow-Origin header myself to a value other than the server name.

This pull request adds an environment variable, CORS_ORIGIN, and documents it in the README. If unset, this value will default to that of SERVER_NAME to maintain backward compatibility. Otherwise, it allows overriding just the origin header.

I hope this change helps other people in my situation!

[arel]

Actually, I believe add_header does not override but rather appends a header. So, I added a line to first hide the server's header, if any, since there can only be one.

proxy_hide_header 'Access-Control-Allow-Origin';
add_header 'Access-Control-Allow-Origin' ${CORS_ORIGIN} always;

See: https://serverfault.com/questions/928912/how-do-i-remove-a-server-added-header-from-proxied-location
See: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS/Errors/CORSMultipleAllowOriginNotAllowed

[justsml]

Thanks @arel !!!
Sorry for the delay!

You're PR's look great, I'll review & merge em in, thanks!