#### Summary

I can not make charset work. I am making request for a page that deals with `charset=iso-8859-1`, and even setting `{ charset: 'latin1' }`, correct decoding does not happen.

I'm following this https://github.com/axios/axios/pull/869

I've tried `latin1` and `latin-1`

```javascript
  // url to open bases results
  const url = `http://www.stf.jus.br/portal/jurisprudencia/listarJurisprudencia.asp?s1=%28SAUDE+COMPLEMENTAR%29&base=baseAcordaos&url=http://tinyurl.com/y9e68odp`

  // request first page
  axios.get(url, {}, { charset: 'latin-1' })
    .then(({ data }) => {
      console.log(data)
    })
```

#### Context

- axios version: *v0.18.0*
- Environment: *node v10.1.0*

Thank you

lib/core/mergeConfig.js
config[prop] = utils.deepMerge(config1[prop]);
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });
  utils.forEach(defaultToConfig2Keys, function defaultToConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });
  var axiosKeys = valueFromConfig2Keys
    .concat(mergeDeepPropertiesKeys)
    .concat(defaultToConfig2Keys);
  var otherKeys = Object
    .keys(config2)
    .filter(function filterAxiosKeys(key) {
      return axiosKeys.indexOf(key) === -1;
    });
  utils.forEach(otherKeys, function otherKeysDefaultToConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });
  return config;
};