Fix vp8 keyframe detection

[Karolk99]

Closes #234

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.25%. Comparing base (cdae334) to head (120a56d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
+ Coverage   88.06%   88.25%   +0.18%     
==========================================
  Files          59       59              
  Lines        2808     2809       +1     
==========================================
+ Hits         2473     2479       +6     
+ Misses        335      330       -5     

Files with missing lines	Coverage Δ
lib/ex_webrtc/rtp/vp8.ex	100.00% <100.00%> (+100.00%)	⬆️

... and 3 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cdae334...120a56d. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.