geocode function failed when involving bad request among multiple location inputs

[zhiruiwang]

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

library(ggmap)
geocode(c("CASIRATE  D\u0092ADDA Italy","Shanghai"),output = "more")

Get the following warning:

Error in list_to_dataframe(res, attr(.data, "split_labels"), .id, id_as_factor) : 
  Results must be all atomic, or all data frames

[zhiruiwang]

The problem is failedGeocodeReturn function returns a named vector instead of a data.frame, this will cause problem when ldply function wants to combine named vectors and data.frames.
Already created a PR to solve this problem:#219