Aserednyakov fix legacy files regression

[scottbelden]

Resolves #560

[codecov]

Codecov Report

Merging #561 (a155dc7) into master (e4ce523) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
+ Coverage   98.49%   98.50%   +0.01%     
==========================================
  Files          27       27              
  Lines        2057     2071      +14     
==========================================
+ Hits         2026     2040      +14     
  Misses         31       31              

Impacted Files	Coverage Δ
fastavro/_write_py.py	98.57% <ø> (ø)
fastavro/_schema_py.py	98.63% <100.00%> (+<0.01%)	⬆️
fastavro/io/binary_encoder.py	100.00% <0.00%> (ø)

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