remove Content-Type requirement

* https://github.com/golang/go/issues/31753 is fixed

http.go

@@ -11,15 +11,7 @@ import (
 // the Accept-Encoding header, sets the Content-Encoding header, and returns a
 // WriteCloser that implements that compression. The Close method must be called
 // before the current HTTP handler returns.
-//
-// Due to https://github.com/golang/go/issues/31753, the response will not be
-// compressed unless you set a Content-Type header before you call
-// HTTPCompressor.
 func HTTPCompressor(w http.ResponseWriter, r *http.Request) io.WriteCloser {
-	if w.Header().Get("Content-Type") == "" {
-		return nopCloser{w}
-	}
-
 	if w.Header().Get("Vary") == "" {
 		w.Header().Set("Vary", "Accept-Encoding")
 	}