Use known "scheme" zend_string instead of C string

Author: Girgias

ext/iconv/iconv.c

@@ -1978,7 +1978,7 @@ PHP_FUNCTION(iconv_mime_encode)
 	if (pref != NULL) {
 		zval *pzval;
 
-		if ((pzval = zend_hash_str_find_deref(Z_ARRVAL_P(pref), "scheme", sizeof("scheme") - 1)) != NULL) {
+		if ((pzval = zend_hash_find_deref(Z_ARRVAL_P(pref), ZSTR_KNOWN(ZEND_STR_SCHEME)) != NULL) {
 			if (Z_TYPE_P(pzval) == IS_STRING && Z_STRLEN_P(pzval) > 0) {
 				switch (Z_STRVAL_P(pzval)[0]) {
 					case 'B': case 'b':