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[tinovyatkin]

In sync with node-fetch

1. Tests on more OSes and with the minimum required Node.js version
2. Switch to native V8 code coverage via c8
3. Enable partial line coverage for Codecov
4. Fixes small bug in test on Node 10.17

[codecov]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            master       #47    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            1         1            
  Lines           70       183   +113     
  Branches        20        30    +10     
==========================================
+ Hits            70       183   +113     

Impacted Files	Coverage Δ
index.js	100.00% <0.00%> (ø)

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