fix unpinning window to new workspace (#278)

src/workspace.cpp

@@ -359,7 +359,7 @@ void Workspace::transfer_pinned_windows_to(std::shared_ptr<Workspace> const& oth
         auto floating = Container::as_floating(container);
         if (floating && floating->pinned())
         {
-            other->floating_windows.push_back(floating);
+            other->graft(floating);
             it = floating_windows.erase(it);
         }
         else