Fix#282, which went stale over 7 years ago, and is now reopened. This is certainly related to issue #4379. If merged cells are in the midst of deleted rows/columns, the merge may continue to exist after the deletion, leading to various problems including spreadsheet corruption. The problem is not with either Reader or Writer.
This PR will automatically unmerge all merged cells whose first cell is in the delete range before performing the deletion. This will address the corruption problem. Something more sophisticated may be required, so I won't merge this PR for a while to give me a chance to think about it some more.