Set detected Content-Type only when not already set

gzip.go

@@ -128,6 +128,12 @@ func (w *GzipResponseWriter) Write(b []byte) (int, error) {
 			// If a Content-Type wasn't specified, infer it from the current buffer.
 			if ct == "" {
 				ct = http.DetectContentType(w.buf)
+			}
+
+			// Handles the intended case of setting a nil Content-Type (as for http/server or http/fs)
+			// Set the header only if the key does not exist
+			_, haveType := w.Header()["Content-Type"]
+			if !haveType {
 				w.Header().Set(contentType, ct)
 			}
 			// If the Content-Type is acceptable to GZIP, initialize the GZIP writer.

gzip_test.go

@@ -81,6 +81,21 @@ func TestGzipHandler(t *testing.T) {
 	assert.Equal(t, http.DetectContentType([]byte(testBody)), res3.Header().Get("Content-Type"))
 }
 
+func TestGzipHandlerNilContentType(t *testing.T) {
+	// This just exists to provide something for GzipHandler to wrap.
+	handler := newTestHandler(testBody)
+
+	// content-type header not set when provided nil
+
+	req, _ := http.NewRequest("GET", "/whatever", nil)
+	req.Header.Set("Accept-Encoding", "gzip")
+	res := httptest.NewRecorder()
+	res.Header()["Content-Type"] = nil
+	handler.ServeHTTP(res, req)
+
+	assert.Empty(t, res.Header().Get("Content-Type"))
+}
+
 func TestGzipHandlerSmallBodyNoCompression(t *testing.T) {
 	handler := newTestHandler(smallTestBody)