#### 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
* by merging two configuration objects together.
 *
 * @param {Object} config1
 * @param {Object} config2
 * @returns {Object} New object resulting from merging config2 to config1
 */
module.exports = function mergeConfig(config1, config2) {
  
  config2 = config2 || {};
  var config = {};
  var valueFromConfig2Keys = ['url', 'method', 'data'];
  var mergeDeepPropertiesKeys = ['headers', 'auth', 'proxy', 'params'];
  var defaultToConfig2Keys = [
    'baseURL', 'url', 'transformRequest', 'transformResponse', 'paramsSerializer',
    'timeout', 'withCredentials', 'adapter', 'responseType', 'xsrfCookieName',
    'xsrfHeaderName', 'onUploadProgress', 'onDownloadProgress',
    'maxContentLength', 'validateStatus', 'maxRedirects', 'httpAgent',
    'httpsAgent', 'cancelToken', 'socketPath'
  ];
  utils.forEach(valueFromConfig2Keys, function valueFromConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    }
  });
  utils.forEach(mergeDeepPropertiesKeys, function mergeDeepProperties(prop) {
    if (utils.isObject(config2[prop])) {
      config[prop] = utils.deepMerge(config1[prop], config2[prop]);
    } else if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (utils.isObject(config1[prop])) {