Set default 'Server' header only if it isn't set.

Closes #996.

core/lib/src/rocket.rs

@@ -216,9 +216,12 @@ impl Rocket {
         // Route the request and run the user's handlers.
         let mut response = self.route_and_process(request, data);
 
-        // Add the 'rocket' server header to the response and run fairings.
+        // Add the 'rocket' server header to the response and run fairings only if the header
+        // doesn't already exist.
         // TODO: If removing Hyper, write out `Date` header too.
+        if !response.headers().contains("Server") {
             response.set_header(Header::new("Server", "Rocket"));
+        }
         self.fairings.handle_response(request, &mut response);
 
         // Strip the body if this is a `HEAD` request.

core/lib/tests/conditionally-set-server-header-997.rs

@@ -0,0 +1,43 @@
+#![feature(proc_macro_hygiene, decl_macro)]
+
+#[macro_use]
+extern crate rocket;
+extern crate rocket_http;
+
+use rocket::response::Redirect;
+use rocket::Response;
+use rocket_http::{Header, Status};
+
+#[get("/do_not_overwrite")]
+fn do_not_overwrite<'r>() -> Result<Response<'r>, ()> {
+    let header = Header::new("Server", "Test");
+
+    Response::build()
+        .header(header)
+        .ok()
+}
+
+#[get("/use_default")]
+fn use_default<'r>() -> Result<Response<'r>, ()> {
+    Response::build()
+        .ok()
+}
+
+mod conditionally_set_server_header {
+    use super::*;
+    use rocket::local::Client;
+
+    #[test]
+    fn do_not_overwrite_server_header() {
+        let rocket = rocket::ignite().mount("/", routes![do_not_overwrite, use_default]);
+        let client = Client::new(rocket).unwrap();
+
+        let response = client.get("/do_not_overwrite").dispatch();
+        let server = response.headers().get_one("Server");
+        assert_eq!(server, Some("Test"));
+
+        let response = client.get("/use_default").dispatch();
+        let server = response.headers().get_one("Server");
+        assert_eq!(server, Some("Rocket"));
+    }
+}