- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.4
* Operating System version: Darwin Kernel Version 18.7.0
* Java version: 1.8.0_181-b13)

### Steps to reproduce this issue

1. invoke `StringUtils.split("d,1,2,4", 'a')`
2. invoke `"d,1,2,4".split("a")`

Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

`StringUtils.split("d,1,2,4", 'a')` should return `["d,1,2,4"]` just like `"d,1,2,4".split("a")`.

### Actual Result

empty array.

Map<String, String> map = new HashMap<String, String>();
        map.put("key1", "value1");
        map.put("key2", "value2");
        String queryString = StringUtils.toQueryString(map);
        assertThat(queryString, containsString("key1=value1"));
        assertThat(queryString, containsString("key2=value2"));
    }
    @Test
    public void testJoin() throws Exception {
        String[] s = {"1", "2", "3"};
        assertEquals(StringUtils.join(s), "123");
        assertEquals(StringUtils.join(s, ','), "1,2,3");
        assertEquals(StringUtils.join(s, ","), "1,2,3");
    }
    @Test
    public void testSplit() throws Exception {
        String s = "d,1,2,4";
        assertEquals(StringUtils.split(s, ',').length, 4);
    }
    @Test
    public void testTranslate() throws Exception {
        String s = "16314";
        assertEquals(StringUtils.translate(s, "123456", "abcdef"), "afcad");
        assertEquals(StringUtils.translate(s, "123456", "abcd"), "acad");
    }
    @Test
    public void testIsContains() throws Exception {
        assertThat(StringUtils.isContains("a,b, c", "b"), is(true));
        assertThat(StringUtils.isContains("", "b"), is(false));
        assertThat(StringUtils.isContains(new String[]{"a", "b", "c"}, "b"), is(true));