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', $mbstring_list);
             }
@@ -229,14 +231,25 @@ class rcube_charset
         // convert charset using mbstring module
         if ($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)) {
-                    return $out;
+                    // 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;
+                    }
                 }
             }
         }
@@ -646,14 +659,14 @@ class rcube_charset
             return $failover;
         }
 
-        // FIXME: the order is important, because sometimes 
+        // FIXME: the order is important, because sometimes
         // iso string is detected as euc-jp and etc.
         $enc = array(
             'UTF-8', 'SJIS', 'BIG5', 'GB2312',
             'ISO-8859-1', 'ISO-8859-2', 'ISO-8859-3', 'ISO-8859-4',
             'ISO-8859-5', 'ISO-8859-6', 'ISO-8859-7', 'ISO-8859-8', 'ISO-8859-9',
             'ISO-8859-10', 'ISO-8859-13', 'ISO-8859-14', 'ISO-8859-15', 'ISO-8859-16',
-            'WINDOWS-1252', 'WINDOWS-1251', 'EUC-JP', 'EUC-TW', 'KOI8-R', 
+            'WINDOWS-1252', 'WINDOWS-1251', 'EUC-JP', 'EUC-TW', 'KOI8-R',
             'ISO-2022-KR', 'ISO-2022-JP'
         );