Update go test case and update readme.

README.md

@@ -36,11 +36,7 @@ import (
 )
 
 func main() {
-    xlsx, err := excelize.CreateFile()
-    if err != nil {
-        fmt.Println(err)
-        os.Exit(1)
-    }
+    xlsx := excelize.CreateFile()
     xlsx.NewSheet(2, "Sheet2")
     xlsx.NewSheet(3, "Sheet3")
     xlsx.SetCellInt("Sheet2", "A23", 10)

excelize_test.go

@@ -31,6 +31,8 @@ func TestExcelize(t *testing.T) {
 	f1.SetCellValue("Sheet2", "G1", []byte("World"))
 	f1.SetCellValue("Sheet2", "F2", 42)
 	f1.SetCellValue("Sheet2", "G2", nil)
+	// Test completion column.
+	f1.SetCellValue("Sheet2", "M2", nil)
 	// Test read cell value with given axis large than exists row.
 	f1.GetCellValue("Sheet2", "E13")