FIX: multi-byte character broken bug fix

multi-byte character broken 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); 
var buf2 = new Buffer(7);

buffer.copy(buf1, 0, 0, 7); // 'пла' and first byte of 'н' copied to buf1
buffer.copy(buf2, 0, 7, 14); // second byte of 'н' and 'ета' copied to buf2

var str = '';
str += buf1; // This code breaks multi-bytes characters.
str += buf2;  

console.log(str); 
console.log(buffer.toString());
```

result is
```
14
пла��ета
планета
```

The same problem exists in function updateRequest & queryRequest of solr.js.
Please check my commit.
I referred to some codes of "request module"
(https://github.com/mikeal/request/blob/master/request.js)

Thank you.