Add <tbody> tags to html output

[krowbar]

Resolves jazzband/tablib#538

Wrap data rows in <tbody> tags to fix minifying (with a header row) when dropping optional end tags. Otherwise the entire table renders with <thead> styling.

[claudep]

Looks like tests need to be adapted.

[codecov[bot]]

Codecov Report

Merging #539 (8d5e28b) into master (8b9cf49) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
+ Coverage   91.13%   91.15%   +0.01%     
==========================================
  Files          28       28              
  Lines        2674     2680       +6     
==========================================
+ Hits         2437     2443       +6     
  Misses        237      237              

Impacted Files	Coverage Δ
src/tablib/formats/_html.py	100.00% <100.00%> (ø)
tests/test_tablib.py	98.66% <100.00%> (+<0.01%)	:arrow_up:

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