Fixed problem witho doubled 'Accept:' header

I found that if the "Accept" header parameter was added into 'url-request-extra-headers' variable than url package passed to machine:

```
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 take default value (i.e. sparql/xml ). The accepted mime type should be configured by setting up proper value into `url-mime-accept-string` variable.

Regards,