fix bug that overwrites last error when context is canceled

Author: KeiichiHirobe

retry.go

@@ -155,6 +155,7 @@ func Do(retryableFunc RetryableFunc, opts ...Option) error {
 				if config.lastErrorOnly {
 					return config.context.Err()
 				}
+				n++
 				errorLog[n] = config.context.Err()
 				return errorLog
 			}

retry_test.go

@@ -364,7 +364,8 @@ func TestContext(t *testing.T) {
 
 		expectedErrorFormat := `All attempts fail:
 #1: test
-#2: context canceled`
+#2: test
+#3: context canceled`
 		assert.Equal(t, expectedErrorFormat, err.Error(), "retry error format")
 		assert.Equal(t, 2, retrySum, "called at most once")
 	})