Fix crash on send error and keepalive response on pruned pair, release 0.16.0

[sgfn]

two fixes included

• :ok = matches for transport_mod.send on several TURN messages
• crash on keepalive response on pruned pair

[codecov]

Codecov Report

❌ Patch coverage is 78.26087% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.57%. Comparing base (b0de12e) to head (d5e8112).

Files with missing lines	Patch %	Lines
lib/ex_ice/priv/ice_agent.ex	81.57%	7 Missing ⚠️
test/support/transport/mock.ex	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   87.11%   87.57%   +0.45%     
==========================================
  Files          27       27              
  Lines        2127     2140      +13     
==========================================
+ Hits         1853     1874      +21     
+ Misses        274      266       -8     

Files with missing lines	Coverage Δ
test/support/transport/mock.ex	79.54% <62.50%> (-4.24%)	⬇️
lib/ex_ice/priv/ice_agent.ex	89.76% <81.57%> (+0.88%)	⬆️

... and 1 file with indirect coverage changes

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