Fix: Using VCR with Guzzle results in duplicated Host headers.

Using php-vcr in conjunction with Guzzle results in php-vcr duplicating the host header. ``` \VCR\Util\CurlHelper::setCurlOptionOnRequest```  sets it during the ``` CURLOPT_URL ``` and ``` CURLOPT_HTTPHEADER ``` cases.  

Example:
```
\VCR\VCR::configure()->enableLibraryHooks(array('curl_rewrite'));
\VCR\VCR::turnOn();
\VCR\VCR::insertCassette('Demo');

$client = new Guzzle\Http\Client();
$request = $client->get('http://www.google.com');
$response = $request->send();
```

The host header that gets sent over the wire in the above example is:
``` Host: www.google.com, www.google.com\r\n ```