Had a look in the curl base library, its considered a shortcut: http://curl.haxx.se/libcurl/c/curl_easy_setopt.html
Not one we'd want to replicate for our own API, I think - its just a header like everything else. 
Headers shouldn't be associative, since they can be repeated (quite common with the "Link" response header for example, have a look at the github API). But that just means we need a third `$replace=false` argument which looks through existing headers. I agree that its beyond the scope of what you're trying to achieve, but also want to keep the API baseline consistent.