Return io.EOF from closed udpMuxedConn

[paulwe]

write returns io.ErrClosedPipe but read returns io.EOF...

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 79.15%. Comparing base (edb6929) to head (68a02a4).

Files	Patch %	Lines
udp_muxed_conn.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #692      +/-   ##
==========================================
- Coverage   79.21%   79.15%   -0.06%     
==========================================
  Files          41       41              
  Lines        3733     3733              
==========================================
- Hits         2957     2955       -2     
- Misses        547      548       +1     
- Partials      229      230       +1     

Flag	Coverage Δ
go	79.15% <50.00%> (-0.06%)	⬇️
wasm	22.16% <0.00%> (ø)

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