Clear previous output when openToBrowser() called (#243)

If any character has been outputted before, the generated file will be corrupted.

src/Spout/Common/Helper/GlobalFunctionsHelper.php

@@ -252,6 +252,17 @@ class GlobalFunctionsHelper
         header($string);
     }
 
+    /**
+     * Wrapper around global function ob_end_clean()
+     * @see ob_end_clean()
+     *
+     * @return void
+     */
+    public function ob_end_clean()
+    {
+        ob_end_clean();
+    }
+
     /**
      * Wrapper around global function iconv()
      * @see iconv()