ggplot: themes: fix marker default shape

ggplot: themes: fix marker default shape

By changing `shape="o"` from `circle` to `circle-open`, #114 changed default marker shape... Thanks to @mkcor for finding this.

- Change marker default shape to 16
- Add a test

Before PR: https://plot.ly/~pdespouy/1730/illn-vs-edu/;
After PR: https://plot.ly/~pdespouy/1731/illn-vs-edu/

cc/ @mkcor @chriddyp