Update message hash

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

@@ -52,6 +52,7 @@ import java.io.OutputStream;
 import java.io.UnsupportedEncodingException;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.security.NoSuchAlgorithmException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -1182,6 +1183,15 @@ public class MessageHelper {
         return sb.toString();
     }
 
+    String getHash() throws MessagingException {
+        try {
+            return Helper.sha1(getHeaders().getBytes());
+        } catch (NoSuchAlgorithmException ex) {
+            Log.e(ex);
+            return null;
+        }
+    }
+
     static String formatAddresses(Address[] addresses) {
         return formatAddresses(addresses, true, false);
     }