geocode function failed when involving bad request among multiple location inputs

When using geocode function, if:
(1) the location parameter is a multiple-element vector,
(2) one of the element will return a bad request,
(3) output parameter is "more"
Then the whole request will fail

```r
library(ggmap)
geocode(c("CASIRATE D\u0092ADDA Italy","Shanghai"),output = "more")
```
Get the following warning:
```r
Error in list_to_dataframe(res, attr(.data, "split_labels"), .id, id_as_factor) :
Results must be all atomic, or all data frames
```