write_html: do not style header row if no <th> is present - fix #740

[Lucas-C]

This PR also introduces some tests files that I wrote in #703 but forgot to git add before... 🤦‍♂️

[codecov]

Codecov Report

Patch coverage: 30.00% and project coverage change: -0.09 ⚠️

Comparison is base (393580e) 92.67% compared to head (36a8fe4) 92.58%.

❗ Current head 36a8fe4 differs from pull request most recent head ef1c7d7. Consider uploading reports for the commit ef1c7d7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #766      +/-   ##
==========================================
- Coverage   92.67%   92.58%   -0.09%     
==========================================
  Files          27       27              
  Lines        7275     7283       +8     
  Branches     1317     1320       +3     
==========================================
+ Hits         6742     6743       +1     
- Misses        337      341       +4     
- Partials      196      199       +3     

Impacted Files	Coverage Δ
fpdf/html.py	86.13% <22.22%> (-1.49%)	⬇️
fpdf/table.py	84.83% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.