Add default cell style definition

[artiz]

Hi, I have found that XLSX file generated by your library is opened with following warning in openpyxl:

.local/lib/python3.8/site-packages/openpyxl/styles/stylesheet.py:226: UserWarning: Workbook contains no default style, apply openpyxl's default                                                       
warn("Workbook contains no default style, apply openpyxl's default")   

I checked empty Excel file file structure and found that it has cellStyleXfs/cellStyles elements with default "Normal" style.
So I have added same definitions to workbook builder.
With these changes tha warning is elminated.

Also I have fixed logic bug with contentTypesAdded used for Content-Types duplication check

[arthurblake-AngelOak]

Unfortunately It looks like this excellent project might be abandoned by the author...
I hope @natergj is alive and well and on to better things.
I think our best bet going forward is this fork: https://www.npmjs.com/package/@advisr/excel4node
I asked @jrohland nicely to merge this PR and it's done!
See advisr-io@7794572