Spreadsheet - Fix content_expression to be updated properly
Previously, the content expression wasn't updated when a cell's value/expression was updated, but stayed as the first value given to it. This resulted in unexpected behavior where the cell's value wouldn't update when the expression was updated.
E.g. if a cell's expression was "=1+1" and changed to "=2+2" it would show 4 correctly, but changing back to "=1+1" wouldn't resolve as the expression was equal to the cell's already existing expression, which was always the first one given (in this case "=1+1"). The expression should be properly updated.