Figures need to be "boxed" #341

Consider what happens when I try to obtain and print this plot

get_figure("cpsievert", "558")

That's because we're currently not "boxing special properties" in the figure object. That is, these fields to have an AsIs class attached so our JSON converter knows they should be arrays of length one as opposed to scalars

f <- get_figure("cpsievert", "558")
l <- plotly_build(f)
l$data[[3]][c('x', 'y')]
$x
[1] "0"

$y
[1] 14