fix serialize String with '\n' unescaped, for issue #1387

core/src/main/java/com/alibaba/fastjson2/JSONWriterUTF16.java

@@ -2583,7 +2583,7 @@ public final void writeString(final char[] chars) {
         boolean special = false;
         for (int i = 0; i < chars.length; ++i) {
             char c = chars[i];
-            if (c == '\\' || c == quote) {
+            if (c == '\\' || c == quote || c < ' ') {
                 special = true;
                 break;
             }

core/src/test/java/com/alibaba/fastjson2/issues_1000/Issue1387.java

@@ -0,0 +1,28 @@
+package com.alibaba.fastjson2.issues_1000;
+
+import com.alibaba.fastjson2.JSON;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue1387 {
+    @Test
+    public void test() {
+        String json = "{\"date\":\"2021\\nNOV\"}";
+        Bean params = JSON.parseObject(json, Bean.class);
+        assertEquals(json, JSON.toJSONString(params));
+        assertEquals(json, new String(JSON.toJSONBytes(params)));
+    }
+
+    public static class Bean {
+        private String date;
+
+        public String getDate() {
+            return date;
+        }
+
+        public void setDate(String date) {
+            this.date = date;
+        }
+    }
+}