Support nested heartbeat response status in 2xx range

[alanhamlett]

The HTTP response needs 201 or 202 because some proxies or other network configurations always return 200 status even for failed requests that don't reach the WakaTime API. Because the WakaTime API never responds to POST heartbeats with 200 status, we want to treat 200 status as an error and save the heartbeats to the local offline db.

But for the nested JSON array in the response content we can accept any 2xx status code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.11%. Comparing base (d32ccc3) to head (3d05897).
Report is 2 commits behind head on develop.

@@             Coverage Diff             @@
##           develop    #1106      +/-   ##
===========================================
- Coverage    63.11%   63.11%   -0.01%     
===========================================
  Files          383      383              
  Lines        16565    16564       -1     
===========================================
- Hits         10455    10454       -1     
  Misses        5541     5541              
  Partials       569      569              

Flag	Coverage Δ
unittests	63.11% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/api/heartbeat.go	79.06% <100.00%> (ø)
pkg/offline/offline.go	55.61% <100.00%> (-0.13%)	⬇️