Support additional headers

Author: johnd0e

openai/init.moon

@@ -381,6 +381,10 @@ class OpenAI
       "Authorization": "Bearer #{@api_key}"
     }
 
+    if @headers
+      for k,v in pairs @headers
+        headers[k] = v
+
     if more_headers
       for k,v in pairs more_headers
         headers[k] = v