Change Create BatchSendingItem endpoint from POST to PUT

[IamDejv]

Create Endpoint now overwrite all items in BatchSending

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.54%. Comparing base (106cad9) to head (7254bad).
Report is 3 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #267   +/-   ##
=========================================
  Coverage     97.54%   97.54%           
- Complexity      468      469    +1     
=========================================
  Files            74       74           
  Lines          1100     1102    +2     
=========================================
+ Hits           1073     1075    +2     
  Misses           27       27           

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