items_create ignores multiple parents

[aappling-usgs]

I want the following code to create the following items under user_id():

* bigitem
  * item 1
  * item 2
* top-level item

Here's the code:

new_folder <- item_create(title='bigitem')
add_mult <- items_create(
	parent_id = c(new_folder$id, new_folder$id, user_id()),
	title = c("item 1", "item 2", "top-level item"))

What I get is three items nested under bigitem instead of bigitem and 'top-level item' being siblings:

* bigitem
  * item 1
  * item 2
  * top-level item

I think this is a bug, and I think it's due to a faulty else option in this line: https://github.com/USGS-R/sbtools/blob/master/R/items_create.R#L61. Rather than the current line,

item <- if (length(item) == 1) rep(item[[1]]$id, length(title)) else item[[1]]$id 

I suggest this:

item <- if (length(item) == 1) rep(item[[1]]$id, length(title)) else sapply(item, function(i) i$id) 

followed by some testing to make sure the change actually permits multiple parents for multiple items.

[lindsayplatt]

used in the DL curriculum, so would be good to fix

[aappling-usgs]

Seems reasonable to me. @lindsaycarr , you up for taking this one?