IMAP: downloading mails is now working.

* Changed the way the attributes are written to make sure that everything
  that can be written once is in fact written just once.
* The rename code in BMailProtocol::_ProcessFetchedHeader() was broken,
  and caused the hang of the last commit.

headers/os/app/Message.h

@@ -489,7 +489,8 @@ public:
 			status_t			SetRect(const char* name, const BRect& value);
 			status_t			SetSize(const char* name, const BSize& value);
 			status_t			SetData(const char* name, type_code type,
-									const void* data, ssize_t numBytes);
+									const void* data, ssize_t numBytes,
+									bool fixedSize = true, int count = 1);
 
 	class Private;
 	struct message_header;