Guess convert plain text to UTF8

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -1729,6 +1729,11 @@ public class MessageHelper {
                     warnings.add(context.getString(R.string.title_no_charset, charset));
 
                 if (part.isMimeType("text/plain")) {
+                    if (Helper.isUTF8(result)) {
+                        Log.i("Charset plain=UTF8");
+                        result = new String(result.getBytes(StandardCharsets.ISO_8859_1), StandardCharsets.UTF_8);
+                    }
+
                     if ("flowed".equalsIgnoreCase(ct.getParameter("format")))
                         result = HtmlHelper.flow(result);
                     result = "<div x-plain=\"true\">" + HtmlHelper.formatPre(result) + "</div>";