[bug fix] breaking multi-byte characters

breaking multi-byte character bug fix

Let me show you some code 
```
var buffer = new Buffer('планета'); // some cyrillic characters here
console.log(buffer.length);
var buf1 = new Buffer(7); // cut between 2-byte character
var buf2 = new Buffer(7);

buffer.copy(buf1, 0, 0, 7);
buffer.copy(buf2, 0, 7, 15);

var str = '';
str += buf1;
str += buf2;

console.log(str);
console.log(buffer.toString());
```

result is
```
14
пла��ета
планета
```

the same problem is exist in solr.js code line 516.