Fix collapse result to detect the issue

[cderv]

This revert part of the change at 931e0a2a4e273dcdc13902291b6c6b6e448658cb that should have been seen as breakage.

collapse = TRUE should still merge the cells together. but 1.46 introduced breakage : https://github.com/yihui/knitr/issues/2346

Companion PR to

• https://github.com/yihui/knitr/pull/2347