BUG: Fix for extraneous default cell format in xlsxwriter files.
Fix for issue in the xlsxwriter engine where is adds a default
'General' format to cells if no other format is applied. This
isn't a bug, per se, but it prevents other row or column formatting.

closes #9167

@jmcnamara pls add a release note. This is a bug fix? user noticeable? (so an API change?) pls make the appropriate entry. thxs.
 > add a release note. This is a bug fix? user noticeable? (so an API change?) pls make the appropriate entry. thxs.

I'm not sure if it needs a release note entry since it doesn't really affect pandas. It it mainly to allow users to format columns in the encapsulated xlsxwriter object. If you think that it needs an entry I'll add one. Should it go in `doc/source/whatsnew/v0.16.0.txt`?
 it needs a note  - in v0.16.0 bug fix section
 @jreback 
- Added release note.
- Used `_skip_if_no_openpyxl()` as noted above.
- Rebased and re-pushed.
 merged via https://github.com/pydata/pandas/commit/07a573561422c83441b0e4023a73421d495d89f1

thanks!
