#### 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
'use strict';
var utils = require('../utils');
/**
 * Config-specific merge-function which creates a new config-object