- [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.

}
        StringBuilder sb = new StringBuilder();
        boolean isFirst = true;
        for (String s : coll) {
            if (isFirst) {
                isFirst = false;
            } else {
                sb.append(split);
            }
            sb.append(s);
        }
        return sb.toString();
    }
    /**
     * parse key-value pair.
     *
     * @param str           string.
     * @param itemSeparator item separator.
     * @return key-value map;
     */
    private static Map<String, String> parseKeyValuePair(String str, String itemSeparator) {
        String[] tmp = str.split(itemSeparator);
        Map<String, String> map = new HashMap<String, String>(tmp.length);
        for (int i = 0; i < tmp.length; i++) {
            Matcher matcher = KVP_PATTERN.matcher(tmp[i]);
            if (!matcher.matches()) {
                continue;
            }
            map.put(matcher.group(1), matcher.group(2));
        }