fixed race condition on channel close while go routine was sending

[michelvocks]

Fixes #77

[codecov-io]

Codecov Report

Merging #85 into master will decrease coverage by 0.15%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
- Coverage   64.59%   64.44%   -0.16%     
==========================================
  Files          22       22              
  Lines        1881     1887       +6     
==========================================
+ Hits         1215     1216       +1     
- Misses        535      539       +4     
- Partials      131      132       +1

Impacted Files	Coverage Δ
scheduler/scheduler.go	73.57% <83.33%> (-1.25%)	⬇️

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[Skarlso]

Yep. That was my thinking on that too.