fix missing context ID in datagrams

[marten-seemann]

For UDP payloads, the context ID is 0, see section 5 of RFC 9298.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 72.38%. Comparing base (acf6987) to head (44a8db5).
Report is 1 commits behind head on master.

Files	Patch %	Lines
conn.go	60.00%	2 Missing and 2 partials ⚠️
proxy.go	60.00%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   73.66%   72.38%   -1.29%     
==========================================
  Files           4        4              
  Lines         300      315      +15     
==========================================
+ Hits          221      228       +7     
- Misses         56       60       +4     
- Partials       23       27       +4     

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