Bug 1933116 - Fix "Overwriting an existing document channel!" assertion in tests. r=tobyp

Differential Revision: https://phabricator.services.mozilla.com/D230069

--HG--
extra : amend_source : bb001931a73bac04463caa171c852de0620770d2

mailnews/base/src/nsMsgProtocol.cpp

@@ -428,6 +428,7 @@ NS_IMETHODIMP nsMsgProtocol::Open(nsIInputStream** _retval) {
 }
 
 NS_IMETHODIMP nsMsgProtocol::AsyncOpen(nsIStreamListener* aListener) {
+  mLoadFlags |= nsIChannel::LOAD_REPLACE;
   nsCOMPtr<nsIStreamListener> listener = aListener;
   nsresult rv =
       nsContentSecurityManager::doContentSecurityCheck(this, listener);

mailnews/imap/src/nsImapProtocol.cpp

@@ -9559,6 +9559,7 @@ bool nsImapMockChannel::ReadFromLocalCache() {
 NS_IMETHODIMP nsImapMockChannel::AsyncOpen(nsIStreamListener* aListener) {
   MOZ_ASSERT(NS_IsMainThread(),
              "nsIChannel methods must be called from main thread");
+  mLoadFlags |= nsIChannel::LOAD_REPLACE;
   nsCOMPtr<nsIStreamListener> listener = aListener;
   nsresult rv =
       nsContentSecurityManager::doContentSecurityCheck(this, listener);