feat: `transpose` support dataset with no headers

[sgqy]

data.transpose() raises TypeError: 'NoneType' object is not subscriptable, when headers is not set.

[codecov[bot]]

Codecov Report

Merging #560 (fe7d8dd) into master (98b8c53) will increase coverage by 0.10%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   91.64%   91.74%   +0.10%     
==========================================
  Files          28       28              
  Lines        2800     2823      +23     
==========================================
+ Hits         2566     2590      +24     
+ Misses        234      233       -1     

Files Changed	Coverage Δ
src/tablib/core.py	85.71% <100.00%> (+0.56%)	:arrow_up:
tests/test_tablib.py	98.79% <100.00%> (+0.01%)	:arrow_up:

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