Fix to_type_string/1 for 3rd party endpoints

[mickel8]

see #284

[codecov]

Codecov Report

Merging #285 (8c52239) into master (1e87c76) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   62.92%   62.94%   +0.01%     
==========================================
  Files          44       44              
  Lines        2131     2129       -2     
==========================================
- Hits         1341     1340       -1     
+ Misses        790      789       -1     

Impacted Files	Coverage Δ
...embrane_rtc_engine/endpoints/webrtc/media_event.ex	22.85% <0.00%> (+0.63%)	⬆️

... and 4 files with indirect coverage changes

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