Prevent adding Accept header when already one.

transport.go

@@ -119,15 +119,18 @@ func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
 			}
 		}()
 	}
-	
+
 	token, err := t.Token(req.Context())
 	if err != nil {
 		return nil, err
 	}
 
 	creq := cloneRequest(req) // per RoundTripper contract
 	creq.Header.Set("Authorization", "token "+token)
-	creq.Header.Add("Accept", acceptHeader) // We add to "Accept" header to avoid overwriting existing req headers.
+
+	if creq.Header.Get("Accept") == "" { // We only add an "Accept" header to avoid overwriting the expected behavior.
+		creq.Header.Add("Accept", acceptHeader)
+	}
 	reqBodyClosed = true // req.Body is assumed to be closed by the tr RoundTripper.
 	resp, err := t.tr.RoundTrip(creq)
 	return resp, err

transport_test.go

@@ -177,6 +177,18 @@ func TestNew_appendHeader(t *testing.T) {
 	if !found {
 		t.Errorf("could not find %v in request's accept headers: %v", myheader, headers["Accept"])
 	}
+
+	// Here we test that there isn't a second Accept header.
+	// Though the Accept header 'application/vnd.github.v3+json' is used for most
+	//  interactions with the GitHub API, having this header will force the
+	//  GitHub API response as JSON, which we don't want when downloading a
+	//  release (octet-stream)
+	for _, v := range headers["Accept"] {
+		if v == acceptHeader {
+			t.Errorf("accept header '%s' should not be present when accept header '%s' is set: %v", acceptHeader, myheader, headers["Accept"])
+			break
+		}
+	}
 }
 
 func TestRefreshTokenWithParameters(t *testing.T) {