Improvement `remote.request_book_offers` parameters passing

As the example  in doc reference:
```js
var request = remote.request_book_offers({
  gets: {
      'currency':'XRP'
  },
  pays: {
    'currency':'USD',
    'issuer': 'rvYAfWj5gh67oV6fW32ZzP3Aw4Eubs59B'
  }
});
```
It will raise `TypeError: Cannot read property 'currency' of undefined`. 
Fixed issue with parsing parameters