Fix env buffer

This pull request is written after issue #54. After looking at the code, I realized that the buffer offset seems incorrect. After this change, there is no more error and the exec command return. 

However, I would exect the `env` command to print the "http_proxy" in this example:

```js
c = new Connection()
c.on 'ready', () ->
  c.exec 'env', {env:{http_proxy:'test'}}, (err, stream) ->
    stream.on 'data', (data, extended) ->
      console.log data.toString()
    stream.on 'exit', (code, signal) ->
      c.end()
```