[BUG] Prop data_previous isn't correctly updated with copy+paste

[HammadTheOne]

As seen with some of the specific basic copy and paste tests found here, the data_previous prop is not consistently updated when copy and pasting content into a cell.

While the callback fires successfully, the original data of the cell is the same as the data presented with the data. This behaviour is not present when directly inputting content into a cell.

As part of plotly/dash#1679, the copy+paste tests have been marked to be skipped, and must be re-enabled once this issue is resolved.