use utf-8 as default encoding fixes #147

I will leave this pull request in order to fix this ASAP, but as my comment says:

### **original comment from #147**
we are obtaining the charset from the response header:

this is at **GsonRequest.java** line number: **127**
```
    String json = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
```
when we parse the charset from the response headers we obtain: **ISO-8859-1\*** should we start using only utf-8 and not obtaining info from the headers? :hammer: 

**\*** according to [volley documentation](https://android.googlesource.com/platform/frameworks/volley/+/master/src/com/android/volley/toolbox/HttpHeaderParser.java) the method parseCharset returns: ISO-8859-1 as default when no Content-Type is returned in the header, I think we should open a bug at [hub](https://github.com/gdg-x/hub) and fix there but if you ask me... I would prefer to work only in utf-8