fix error in repr_text() on packed tibble

[Fan-iX]

I'm attempting to fix the error in repr_text caused by packed tibbles #154.

Now

iris %>%
  as_tibble() %>%
  pack(
    Sepal = starts_with("Sepal"),
    Petal = starts_with("Petal"),
    .names_sep = "."
  )

should be represented as

  Species Sepal$Length Sepal$Width Petal$Length Petal$Width
1 setosa  5.1          3.5         1.4          0.2        
2 setosa  4.9          3.0         1.4          0.2        
3 setosa  4.7          3.2         1.3          0.2        
4 setosa  4.6          3.1         1.5          0.2        
5 setosa  5.0          3.6         1.4          0.2        
6 setosa  5.4          3.9         1.7          0.4