Accept padded DataChannelAck, fix closing DataChannel

[mickel8]

Pion seems to pad DataChannelAck to 4 bytes: https://github.com/pion/datachannel/blob/master/message_channel_ack.go#L10

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.26%. Comparing base (26f929d) to head (b5b7801).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/sctp_transport.ex	0.00%	1 Missing ⚠️
lib/ex_webrtc/sctp_transport/dcep.ex	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   88.29%   88.26%   -0.04%     
==========================================
  Files          49       49              
  Lines        2580     2581       +1     
==========================================
  Hits         2278     2278              
- Misses        302      303       +1     

Files with missing lines	Coverage Δ
lib/ex_webrtc/sctp_transport.ex	83.23% <0.00%> (ø)
lib/ex_webrtc/sctp_transport/dcep.ex	66.66% <0.00%> (-2.30%)	⬇️

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