Added sendfile support

There seems to be an issue in nginx on VirtualBox/Vagrant when sendfile is set to 'on'. This PR simply allows the user to override the default sendfile setting like this:

```puppet
class { 'nginx': 
	sendfile             => 'off',
	proxy_http_version   => '1.1',
	proxy_set_header     => [
		'Upgrade $http_upgrade',
		'Connection "upgrade"',
		'Host $host'
	]
}
```