rlm_rest: Add additional HTTP Header

[g4-lisz at tonarchiv.ch]

Hi there

The REST API we are using requires an additional HTTP header
"API-Version: 2.0".

The documention in modules-available/rest file says: Additional HTTP
headers may be specified with `control.REST-HTTP-Header`.

The only way I got it working was by putting these lines in the sites
config:

authorize {
    update control {
        &REST-HTTP-Header += "API-Version: 2.0"
    }
}

But then it's added to every request. It would be much nicer if I could
add it to the module config, because it's module-specific. Is there a
way to do this?

Cheers
Till