- [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.

list.add("v1");
        assertEquals("v1", StringUtils.join(list, "-"));
        list.add("v2");
        list.add("v3");
        String out = StringUtils.join(list, ":");
        assertEquals("v1:v2:v3", out);
    }
    @Test
    public void testCamelToSplitName() throws Exception {
        assertEquals("ab-cd-ef", StringUtils.camelToSplitName("abCdEf", "-"));
        assertEquals("ab-cd-ef", StringUtils.camelToSplitName("AbCdEf", "-"));
        assertEquals("ab-cd-ef", StringUtils.camelToSplitName("ab-cd-ef", "-"));
        assertEquals("abcdef", StringUtils.camelToSplitName("abcdef", "-"));
    }
    @Test
    public void testToArgumentString() throws Exception {
        String s = StringUtils.toArgumentString(new Object[]{"a", 0, Collections.singletonMap("enabled", true)});
        assertThat(s, containsString("a,"));
        assertThat(s, containsString("0,"));
        assertThat(s, containsString("{\"enabled\":true}"));
    }
    @Test
    public void testTrim() {
        assertEquals("left blank", StringUtils.trim(" left blank"));
        assertEquals("right blank", StringUtils.trim("right blank "));
        assertEquals("bi-side blank", StringUtils.trim(" bi-side blank "));
    }
    @Test
    public void testToURLKey() {
        assertEquals("dubbo.tag1", StringUtils.toURLKey("dubbo_tag1"));