Rename CI_Utf8::_is_ascii() to is_ascii() and make it public

No reason for it to be protected.
diff --git a/system/core/Utf8.php b/system/core/Utf8.php
index 6ca1a02..98352db 100644
--- a/system/core/Utf8.php
+++ b/system/core/Utf8.php
@@ -73,14 +73,12 @@
 	 *
 	 * Ensures strings contain only valid UTF-8 characters.
 	 *
-	 * @uses	CI_Utf8::_is_ascii()	Decide whether a conversion is needed
-	 *
 	 * @param	string	$str	String to clean
 	 * @return	string
 	 */
 	public function clean_string($str)
 	{
-		if ($this->_is_ascii($str) === FALSE)
+		if ($this->is_ascii($str) === FALSE)
 		{
 			if (ICONV_ENABLED)
 			{
@@ -147,7 +145,7 @@
 	 * @param	string	$str	String to check
 	 * @return	bool
 	 */
-	protected function _is_ascii($str)
+	public function is_ascii($str)
 	{
 		return (preg_match('/[^\x00-\x7F]/S', $str) === 0);
 	}