[fix] pass a base64 string to iconv.convert()

Like the below:
```javascript
var Iconv = require('iconv').Iconv;
var iconv = new Iconv('GB2312', 'UTF-8');
var buf = new Buffer('uMm77rrDsMk=', 'base64');

var result = iconv.convert(buf);
console.log(result.toString('utf8'))
```
It don't return the correct result. Because of the error call for `new Buffer(input)`, the correct way is to do `new Buffer(input, encoding)`, the following code is mine:
```javascript

var Iconv = require('iconv').Iconv;
var iconv = new Iconv('GB2312', 'UTF-8');
var result = iconv.convert('uMm77rrDsMk=', 'base64');
console.log(result.toString('utf8'))
```