add tests for !self.is_len_known and self.is_len_known

[jdelgadoalfonso]

WIP: close first task of the issue #70

[codecov]

Codecov Report

Merging #71 (81783af) into main (dffc530) will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   85.54%   85.90%   +0.35%     
==========================================
  Files          42       42              
  Lines        5950     6036      +86     
==========================================
+ Hits         5090     5185      +95     
+ Misses        860      851       -9     

Impacted Files	Coverage Δ
amqp_serde/src/ser.rs	91.86% <100.00%> (+4.65%)	⬆️

... and 2 files with indirect coverage changes

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[gftea]

thanks!