Allow printing of blobs in regular data.frames

[jimhester]

• blob_size() needs to ignore extra arguments
• digits is set to NULL by print.data.frame(), so this case needs to use the default (3).

A reprex for this (which works after this PR)

iris$blob <- as.blob(lapply(seq_len(nrow(iris)), function(x) as.raw(sample(0:100, size = 
sample(0:25, 1)))))
print(head(iris))
#>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species       blob
#> 1          5.1         3.5          1.4         0.2  setosa blob[12 B]
#> 2          4.9         3.0          1.4         0.2  setosa blob[19 B]
#> 3          4.7         3.2          1.3         0.2  setosa  blob[7 B]
#> 4          4.6         3.1          1.5         0.2  setosa blob[10 B]
#> 5          5.0         3.6          1.4         0.2  setosa blob[18 B]
#> 6          5.4         3.9          1.7         0.4  setosa  blob[4 B]