Made config::standard() implement .write_fixed_array_header() by default

[VictorKoenders]

This will help with confusion when people are using bincode::serde:: as bincode::config::standard() won't give a runtime error.

[codecov]

Codecov Report

Merging #509 (692ed1b) into trunk (21eebcb) will increase coverage by 0.12%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##            trunk     #509      +/-   ##
==========================================
+ Coverage   70.77%   70.90%   +0.12%     
==========================================
  Files          47       47              
  Lines        3169     3159      -10     
==========================================
- Hits         2243     2240       -3     
+ Misses        926      919       -7     

Impacted Files	Coverage Δ
src/config.rs	90.47% <ø> (ø)
tests/utils.rs	39.18% <0.00%> (+2.00%)	⬆️
tests/serde.rs	97.95% <90.90%> (+1.53%)	⬆️
tests/basic_types.rs	99.33% <100.00%> (+<0.01%)	⬆️
tests/issues/issue_474.rs	96.42% <100.00%> (ø)
src/de/impls.rs	78.39% <0.00%> (+0.69%)	⬆️

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