Fix hovermode default logic for stacked scatter traces

[etpinard]

fixes #3619 - so that by default graphs with horizontally stacked scatter traces get the correct hovermode: 'y' default (that is, until #778 in v2 😏 )

cc @plotly/plotly_js note that orientation is only coerced into fullData once per stack. Unnecessary keys are removed here.

[antoinerg]

Codepen before: https://codepen.io/etpinard/pen/eXWXyy
Codepen after: https://codepen.io/antoinerg/pen/LaBNmz

[antoinerg]

Thank you @etpinard for improving the default hovermode for horizontally stacked scatter traces and for the clear test!

💃