Fix broken non-latin characters in body in ASCII (us-ascii) charset (#156)

* Fix broken non-latin characters in body in ASCII (us-ascii) charset

* Move us-ascii check into convertEncoding()

Author: freescout-helpdesk

src/IMAP/Message.php

@@ -702,6 +702,21 @@ public function convertEncoding($str, $from = "ISO-8859-2", $to = "UTF-8") {
         $from = EncodingAliases::get($from);
         $to = EncodingAliases::get($to);
 
+        // We don't need to do convertEncoding() if charset is ASCII (us-ascii):
+        //     ASCII is a subset of UTF-8, so all ASCII files are already UTF-8 encoded
+        //     https://stackoverflow.com/a/11303410
+        // 
+        // us-ascii is the same as ASCII:
+        //     ASCII is the traditional name for the encoding system; the Internet Assigned Numbers Authority (IANA) 
+        //     prefers the updated name US-ASCII, which clarifies that this system was developed in the US and 
+        //     based on the typographical symbols predominantly in use there.
+        //     https://en.wikipedia.org/wiki/ASCII
+        //
+        // convertEncoding() function basically means convertToUtf8(), so when we convert ASCII string into UTF-8 it gets broken.
+        if (strtolower($from) == 'us-ascii' && $to == 'UTF-8') {
+            return $str;
+        }
+
         if (function_exists('iconv') && $from != 'UTF-7' && $to != 'UTF-7') {
             return iconv($from, $to.'//IGNORE', $str);
         } else {