Function encode

encode(content, encoding): Bytes
Encode a utf8 string to a buffer that encoding with GBK, GB18030, GB2132, ISO8859_1 .etc
Parameters
- content: string
  the utf8 string
- encoding: string
  the encoding method: GBK, GB18030, GB2132, ISO8859_1
Returns Bytes
Example
```
import * as iconv from 'iconv';
let result = iconv.encode('hello 中国', 'GBK');
```