Fix invalid Content-Type header when send_format_flowed=false (#1489992)

CHANGELOG

@@ -45,6 +45,7 @@ CHANGELOG Roundcube Webmail
 - Fix errors when using localStorage in Safari's private browsing mode (#1489996)
 - Fix bug where $Forwarded flag was being set even if server didn't support it (#1490000)
 - Fix various iCloud vCard issues, added fallback for external photos (#1489993)
+- Fix invalid Content-Type header when send_format_flowed=false (#1489992)
 
 RELEASE 1.0.2
 -------------

program/lib/Mail/mime.php

@@ -1387,19 +1387,24 @@ class Mail_mime
 
         if ($headers['Content-Type'] == 'text/plain') {
             // single-part message: add charset and encoding
-            $charset = 'charset=' . $this->_build_params['text_charset'];
-            // place charset parameter in the same line, if possible
-            // 26 = strlen("Content-Type: text/plain; ")
-            $headers['Content-Type']
-                .= (strlen($charset) + 26 <= 76) ? "; $charset" : ";$eol $charset";
+            if ($this->_build_params['text_charset']) {
+                $charset = 'charset=' . $this->_build_params['text_charset'];
+                // place charset parameter in the same line, if possible
+                // 26 = strlen("Content-Type: text/plain; ")
+                $headers['Content-Type']
+                    .= (strlen($charset) + 26 <= 76) ? "; $charset" : ";$eol $charset";
+            }
+
             $headers['Content-Transfer-Encoding']
                 = $this->_build_params['text_encoding'];
         } else if ($headers['Content-Type'] == 'text/html') {
             // single-part message: add charset and encoding
-            $charset = 'charset=' . $this->_build_params['html_charset'];
-            // place charset parameter in the same line, if possible
-            $headers['Content-Type']
-                .= (strlen($charset) + 25 <= 76) ? "; $charset" : ";$eol $charset";
+            if ($this->_build_params['html_charset']) {
+                $charset = 'charset=' . $this->_build_params['html_charset'];
+                // place charset parameter in the same line, if possible
+                $headers['Content-Type']
+                    .= (strlen($charset) + 25 <= 76) ? "; $charset" : ";$eol $charset";
+            }
             $headers['Content-Transfer-Encoding']
                 = $this->_build_params['html_encoding'];
         } else {

program/steps/mail/sendmail.inc

@@ -484,15 +484,11 @@ if (preg_match('/[^\x00-\x7F]/', $MAIL_MIME->getTXTBody())) {
     $transfer_encoding = $RCMAIL->config->get('force_7bit') ? 'quoted-printable' : '8bit';
 }
 else {
-    $text_charset      = '';
+    $text_charset      = 'US-ASCII';
     $transfer_encoding = '7bit';
 }
 
 if ($flowed) {
-    if (!$text_charset) {
-        $text_charset = 'US-ASCII';
-    }
-
     $text_charset .= ";\r\n format=flowed";
 }