Subject issue in MMS message

Open a draft MMS message having some subject, some text and any attachment.
Remove the written subject and again save it as draft.  In the messaging
main screen, can still see previously entered subject.

Change-Id: I1a30978b7c06fab253ea5d4da0254c3780d18a62
diff --git a/core/java/com/google/android/mms/pdu/PduPersister.java b/core/java/com/google/android/mms/pdu/PduPersister.java
index 568aeb7..2a1f23a 100644
--- a/core/java/com/google/android/mms/pdu/PduPersister.java
+++ b/core/java/com/google/android/mms/pdu/PduPersister.java
@@ -855,6 +855,8 @@
         if (subject != null) {
             values.put(Mms.SUBJECT, toIsoString(subject.getTextString()));
             values.put(Mms.SUBJECT_CHARSET, subject.getCharacterSet());
+        } else {
+            values.put(Mms.SUBJECT, "");
         }
         
         long messageSize = sendReq.getMessageSize();