Fix p-retry is accepting function not object/promise

[eric4545]

Fixes #311

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

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[codecov]

Codecov Report

Merging #312 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
- Coverage   99.61%   99.53%   -0.08%     
==========================================
  Files          12       12              
  Lines        1294     1295       +1     
==========================================
  Hits         1289     1289              
- Misses          5        6       +1

Impacted Files	Coverage Δ
src/database.js	97.37% <0%> (-0.43%)	⬇️

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[googlebot]

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