Issue with aesthetic mapping

Please note that in the following example `geom_density` is being used as a stand-in for a custom stat/geom that I wrote for internal projects at my office.

``` r
library(ggmap)
bbox <- c(left=-.1, bottom=-.1, right=1.1,top=1.3 )
zm <- calc_zoom(bbox)
map <- get_openstreetmap(bbox, OSM_scale_lookup(zm))
dat <- data.frame(x=runif(100))

# this works
ggmap(map, base_layer = ggplot(dat))+geom_density(aes(x=x), fill="green")
```


``` r
# this produces an error
ggmap(map)+geom_density(aes(x=x),data=dat, fill="green")
```

> Error in eval(expr, envir, enclos) : object 'lat' not found

``` r
# a different error
ggmap(map)+geom_density(aes(x=runif(100)), fill="green")
```

> Error: Aesthetics must be either length 1 or the same as the data (4): x, y

Tested under `R 3.3.1` with `ggmap 2.6.1` on Windows

Start by reproducing the three ggmap and geom_density examples with R 3.3.1 and ggmap 2.6.1, then inspect how ggmap handles aesthetic mappings and supplied data. Done means the reported mapping cases no longer produce the lat, x, or y errors while preserving the expected density overlay.

Written by the indexing model from the issue text.