Set plot_bgcolor to white by default when converting from matplotlib

[robertoffmoura]

Code PR

• [x] I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
• [x] I have added tests or modified existing tests.
• [x] For a new feature, I have added documentation examples (please see the doc checklist as well).
• [x] I have added a CHANGELOG entry if changing anything substantial.
• [x] For a new feature or a change in behavior, I have updated the relevant docstrings in the code.

[emilykl]

Hi @robertoffmoura , the default for layout.plot_bgcolor is already white, do you have an example of where the current default is not working?