Non square maps plot incorrectly using ggmap()

## Reproducible Example

```r
library(ggmap)
register_google(key = as.character(GoogleAPIkey))

#rectangle
map_1<-get_googlemap(center=c(-121.57,38.51), size=c(250, 500), zoom = 12, scale=2, maptype = "satellite", key=GoogleAPIkey )

ggmap(map_1)

```


In the first example, the image appears twice and is compressed along the x-axis. It was downloaded using a rectangle `size=c(250,500)`. Also notice that the longitude axis labels appear as they should.

```r
#square
map_2<-get_googlemap(center=c(-121.57,38.51), size=c(500, 500), zoom = 12, scale=2, maptype = "satellite", key=GoogleAPIkey )

ggmap(map_2)

```


In the second example, the image is downloaded as a square `size=c(500,500)`. It plots correctly.

Reproduce the issue using the ggmap() and get_googlemap() examples with rectangular and square sizes. Trace how ggmap() handles the downloaded image dimensions; done when a non-square map renders once without x-axis compression and retains correct longitude labels.

Written by the indexing model from the issue text.