Multiple Style Arguments Not Working Correctly

See the discussion [here](http://stackoverflow.com/questions/41049782/making-multiple-style-references-in-google-maps-api/41051499?noredirect=1#comment69422098_41051499) on SO, but the gist is that the documentation leads you to believe you can pass a list of style arguments to `ggmap()`, however you are thrown errors and incorrectly generated urls when using a list of style arguments or concatenation.

Rather, you need to create your second, and subsequent style requests, beginning with "&style=" so that the url is generated correctly.

```
style1 <- c(feature = "poi.medical", element = "geometry", color = "red")
style2 <- c("&style=", feature = "poi.park", element = "geometry", color = "blue")

style <- c(style1, style2)

map <- get_googlemap("new york city",
zoom = 12,
maptype = "roadmap",
style = style)

plot(map)
```

All credit goes to SymbolixAU on Stack Overflow for finding a solution to this issue.

Start at the get_googlemap() entry point and reproduce the issue with the style1/style2 example in the report. Inspect how the style argument is converted into the generated URL, then verify that multiple style requests no longer cause errors or malformed URLs and that the example plots successfully.

Written by the indexing model from the issue text.