add setter for mem cache entry so we can clear it out r=sspitzer, sr=mscott

mailnews/base/public/nsIMsgMailNewsUrl.idl

@@ -72,7 +72,7 @@ interface nsIMsgMailNewsUrl : nsIURL {
   attribute boolean msgIsInLocalCache;
   attribute boolean suppressErrorMsgs; // used to avoid displaying biff error messages
 
-  readonly attribute nsICachedNetData memCacheEntry;
+  attribute nsICachedNetData memCacheEntry;
 
 	const unsigned long eCopy     = 0;
 	const unsigned long eMove     = 1;

mailnews/base/util/nsMsgMailNewsUrl.cpp

@@ -598,6 +598,12 @@ NS_IMETHODIMP nsMsgMailNewsUrl::SetFilePath(const char *i_DirFile)
 	return m_baseURL->SetFilePath(i_DirFile);
 }
 
+NS_IMETHODIMP nsMsgMailNewsUrl::SetMemCacheEntry(nsICachedNetData *memCacheEntry)
+{
+  m_memCacheEntry = memCacheEntry;
+  return NS_OK;
+}
+
 NS_IMETHODIMP nsMsgMailNewsUrl:: GetMemCacheEntry(nsICachedNetData **memCacheEntry)
 {
   NS_ENSURE_ARG(memCacheEntry);