Fix tuple struct encoding in serde

[ZoeyR]

Closes #547

[codecov]

Codecov Report

Merging #549 (0a9ca59) into trunk (6ec69a3) will increase coverage by 0.44%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##            trunk     #549      +/-   ##
==========================================
+ Coverage   54.89%   55.33%   +0.44%     
==========================================
  Files          47       48       +1     
  Lines        4201     4223      +22     
==========================================
+ Hits         2306     2337      +31     
+ Misses       1895     1886       -9     

Impacted Files	Coverage Δ
src/features/serde/ser.rs	74.19% <ø> (+5.08%)	⬆️
tests/utils.rs	44.70% <81.25%> (+5.51%)	⬆️
compatibility/src/misc.rs	100.00% <100.00%> (ø)
tests/issues/issue_547.rs	100.00% <100.00%> (ø)
tests/serde.rs	98.07% <100.00%> (+0.11%)	⬆️
src/lib.rs	13.71% <0.00%> (-0.02%)	⬇️
src/features/serde/de_owned.rs	74.61% <0.00%> (+1.74%)	⬆️
src/features/serde/mod.rs	52.38% <0.00%> (+19.04%)	⬆️

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