ggplot: line styles and width

Summary:
- Fix typo in line style mapping (`dotted` -> `dot`);
- Add multiplier (2x) for converting `line size` from ggplot2 to `line width` in Plotly;
- Fix some tests. For example, they were using `dotted` line type - there's no such thing in Plotly; and,
- Some styling.

Line width default was `2`, that means when line size *was not* defined, an `1` in ggplot2 was a `2` in Plotly. Now when size *was* defined it had the same value in Plotly, when it should be 2x - a `1.5` in ggplot2 should be a `3` in Plotly. 
ggplot2: http://www.cookbook-r.com/Graphs/Bar_and_line_graphs_%28ggplot2%29/line-red-dotted-shape.png):
Plotly before: https://plot.ly/~pdespouy/1807
Plotly after: https://plot.ly/~pdespouy/1806