fix refcell borrow_mut error cause channel closed

[Millione]

#13

[codecov]

Codecov Report

Patch coverage: 76.19% and project coverage change: +0.22 🎉

Comparison is base (0324d16) 83.57% compared to head (414015a) 83.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   83.57%   83.80%   +0.22%     
==========================================
  Files          17       17              
  Lines        1924     1920       -4     
==========================================
+ Hits         1608     1609       +1     
+ Misses        316      311       -5     

Impacted Files	Coverage Δ
src/cache/async.rs	85.39% <62.50%> (+0.08%)	⬆️
src/cache/sync.rs	86.54% <83.33%> (-0.36%)	⬇️
src/ttl.rs	81.70% <85.71%> (+5.04%)	⬆️

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[al8n]

LGTM, thanks!