fix: integration with js-libp2p

[vasco-santos]

In the context of integrating gossipsub into js-libp2p libp2p/js-libp2p#365, some problems appeared and this PR addresses them.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   83.18%   83.22%   +0.03%     
==========================================
  Files           8        8              
  Lines         452      453       +1     
==========================================
+ Hits          376      377       +1     
  Misses         76       76

Impacted Files	Coverage Δ
src/pubsub.js	92.56% <100%> (+0.06%)	⬆️
src/heartbeat.js	70.27% <100%> (ø)	⬆️

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[wemeetagain]

LGTM