print.tbl_df should display columns for zero-row output

[wch]

As things work now, no columns names are printed for zero-row output, and this can be confusing for users. It requires using str() just to find out what the columns are. See rstudio/ggvis#293 for an example of this confusion.

as.tbl(mtcars[0, ])
# Source: local data frame [0 x 11]

[hadley]

What do you want it to print?

Source: local data frame [0 x 11]
  mpg cyl disp  hp drat   wt  qsec vs am gear carb

?

[wch]

That output looks reasonable.