register_google fails if accessed via ggmap:: instead of library(ggmap)

`ggmap::register_google(key = myKey)` gives:

> Error in class(options) <- "ggmap_credentials" :
> attempt to set an attribute on NULL

```
library(ggmap)
register_google(key = myKey)
```

works as expected.

> > sessionInfo() # After library(ggmap)
> R version 4.0.0 (2020-04-24)
> Platform: x86_64-apple-darwin17.0 (64-bit)
> Running under: macOS Mojave 10.14.6
>
> Matrix products: default
> BLAS: /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
> LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib
>
> locale:
> [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
>
> attached base packages:
> [1] stats graphics grDevices utils datasets methods base
>
> other attached packages:
> [1] ggmap_3.0.0 ggplot2_3.3.0
>
> loaded via a namespace (and not attached):
> [1] Rcpp_1.0.4.6 rstudioapi_0.11 magrittr_1.5 tidyselect_1.0.0 munsell_0.5.0 colorspace_1.4-1 lattice_0.20-41
> [8] rjson_0.2.20 R6_2.4.1 jpeg_0.1-8.1 rlang_0.4.6 stringr_1.4.0 plyr_1.8.6 httr_1.4.1
> [15] dplyr_0.8.5 tools_4.0.0 grid_4.0.0 gtable_0.3.0 png_0.1-7 withr_2.2.0 ellipsis_0.3.0
> [22] assertthat_0.2.1 tibble_3.0.1 lifecycle_0.2.0 crayon_1.3.4 tidyr_1.0.3 purrr_0.3.4 bitops_1.0-6
> [29] vctrs_0.2.4 glue_1.4.0 sp_1.4-1 stringi_1.4.6 compiler_4.0.0 pillar_1.4.4 RgoogleMaps_1.4.5.3
> [36] scales_1.1.0 pkgconfig_2.0.3