items_create ignores multiple parents

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:
```r
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,
```r
item <- if (length(item) == 1) rep(item[[1]]$id, length(title)) else item[[1]]$id
```
I suggest this:
```r
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.