Non-square maps misalignes pixels

This command
```r
get_googlemap(size = c(360, 420))
```

run from the `tidyup` branch, produces a noisy picture, essentially the same as https://github.com/dkahle/ggmap/issues/17. I fixed it in my branch essentially the same way as it was fixed in #17: https://github.com/lorenzwalthert/ggmap/commit/c6b68ebbb02929b4d628410bfbd0fdff32ac3152

However, this was quick and dirty, maybe this had unexpected side effects.