Fix convertion to/from US-ASCII using mbstring

Define mbstring substitute_character to work as iconv //IGNORE

program/include/rcube_charset.php

@@ -176,6 +176,7 @@ class rcube_charset
     {
         static $iconv_options   = null;
         static $mbstring_list   = null;
+        static $mbstring_sch    = null;
         static $conv            = null;
 
         $to   = empty($to) ? strtoupper(RCMAIL_CHARSET) : $to;
@@ -221,6 +222,7 @@ class rcube_charset
 
         if ($mbstring_list === null) {
             if (extension_loaded('mbstring')) {
+                $mbstring_sch  = mb_substitute_character();
                 $mbstring_list = mb_list_encodings();
                 $mbstring_list = array_map('strtoupper', $mbstring_list);
             }
@@ -229,17 +231,28 @@ class rcube_charset
         // convert charset using mbstring module
         if ($mbstring_list !== null) {
             $aliases['WINDOWS-1257'] = 'ISO-8859-13';
+            // it happens that mbstring supports ASCII but not US-ASCII
+            if (($from == 'US-ASCII' || $to == 'US-ASCII') && !in_array('US-ASCII', $mbstring_list)) {
+                $aliases['US-ASCII'] = 'ASCII';
+            }
 
             $mb_from = $aliases[$from] ? $aliases[$from] : $from;
             $mb_to   = $aliases[$to] ? $aliases[$to] : $to;
 
             // return if encoding found, string matches encoding and convert succeeded
             if (in_array($mb_from, $mbstring_list) && in_array($mb_to, $mbstring_list)) {
-                if (mb_check_encoding($str, $mb_from) && ($out = mb_convert_encoding($str, $mb_to, $mb_from))) {
+                if (mb_check_encoding($str, $mb_from)) {
+                    // Do the same as //IGNORE with iconv
+                    mb_substitute_character('none');
+                    $out = mb_convert_encoding($str, $mb_to, $mb_from);
+                    mb_substitute_character($mbstring_sch);
+
+                    if ($out !== false) {
                         return $out;
                     }
                 }
             }
+        }
 
         // convert charset using bundled classes/functions
         if ($to == 'UTF-8') {