Really actually fix HttpServer::drain().

It turns out I was testing it wrong, and it still doesn't work. :(

c++/src/kj/compat/http.c++

@@ -1072,7 +1072,7 @@ public:
     // Returns whether we can cleanly drain the stream at this point.
     if (onMessageDone != nullptr) return false;
     snarfBufferedLineBreak();
-    return !lineBreakBeforeNextHeader && leftover != nullptr;
+    return !lineBreakBeforeNextHeader && leftover == nullptr;
   }
 
   kj::Promise<kj::ArrayPtr<char>> readMessageHeaders() {