Backport #401 and prepare release of SpecialFunctions 1.8.6

[devmotion]

This PR backports the bug fix in #401 to the release-1.8 branch since SpecialFunctions 1.8.5 is affected by this bug as well.

[codecov]

Codecov Report

Merging #402 (47eb19f) into release-1.8 (5d9b8a8) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           release-1.8     #402   +/-   ##
============================================
  Coverage        93.57%   93.57%           
============================================
  Files               12       12           
  Lines             2770     2770           
============================================
  Hits              2592     2592           
  Misses             178      178           

Flag	Coverage Δ
unittests	93.57% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/beta_inc.jl	92.51% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5d9b8a8...47eb19f. Read the comment docs.