Closes #24: Fix doubled 'Accept:' header

If the `Accept` header parameter is added into the
`url-request-extra-headers` variable then the url package passed to the
server becomes:

```
Accept: */*
User-Agent: URL/Emacs
Content-Type: application/x-www-form-urlencoded
Accept: xxxxxxxx ;; user variable
```

In that case some servers ignore the second `Accept` and just use the
default value (i.e. sparql/xml). The accepted mime type should be
configured by entering the proper value into the
`url-mime-accept-string` variable.