fix(coap-core): add mapping for 204 http status to C205_CONTENT code

[szysas]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.68%. Comparing base (4a7719b) to head (e8171ad).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...-core/src/main/java/com/mbed/coap/packet/Code.java	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #129   +/-   ##
=========================================
  Coverage     92.67%   92.68%           
- Complexity     2078     2079    +1     
=========================================
  Files           135      135           
  Lines          4699     4700    +1     
  Branches        627      627           
=========================================
+ Hits           4355     4356    +1     
  Misses          195      195           
  Partials        149      149           

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