fixbug for computing 'not concating feature' (#896)
### What problem does this PR solve?
When pdfparser call `_naive_vertical_merge` method,there is a "not
concating feature " value by computing difference between `b` and `b_`'s
layoutno ,but actually is `b` and `b`. I think it's a bug, so fix it.
Please check again.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)