use readfile() instead iof fopen() for file download over webdav.

fopen() didn't work for large files on some servers while readfile()
seems to work for all files on all configurations.

lib/connector/sabre/file.php

@@ -42,7 +42,7 @@ class OC_Connector_Sabre_File extends OC_Connector_Sabre_Node implements Sabre_D
 	 */
 	public function get() {
 
-		return OC_Filesystem::fopen($this->path,'r');
+		return OC_Filesystem::readfile($this->path);
 
 	}