Charset text/plain simplification

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

@@ -1884,13 +1884,11 @@ public class MessageHelper {
                 }
 
                 if (h.isPlainText()) {
-                    if (charset == null || StandardCharsets.ISO_8859_1.equals(cs)) {
+                    if (StandardCharsets.ISO_8859_1.equals(cs) && CharsetHelper.isUTF8(result)) {
-                        Charset detected = CharsetHelper.detect(result);
-                        if (StandardCharsets.ISO_8859_1.equals(cs) &&
-                                StandardCharsets.UTF_8.equals(detected)) {
                         Log.i("Charset upgrade=UTF8");
                         result = new String(result.getBytes(StandardCharsets.ISO_8859_1), StandardCharsets.UTF_8);
-                        } else {
+                    } else if (charset == null) {
+                        Charset detected = CharsetHelper.detect(result);
                         if (detected == null) {
                             if (CharsetHelper.isUTF8(result)) {
                                 Log.i("Charset plain=UTF8");
@@ -1901,7 +1899,6 @@ public class MessageHelper {
                             result = new String(result.getBytes(StandardCharsets.ISO_8859_1), detected);
                         }
                     }
-                    }
 
                     if ("flowed".equalsIgnoreCase(h.contentType.getParameter("format")))
                         result = HtmlHelper.flow(result);