Merge Ellint2 bug fix to master

[mborland]

@jzmaddock since this ellint bug was reported by @StephanTLavavej I've cherry picked your fixes to merge into master. I think it's better we ask permission to merge in the beta window rather than wait another 4ish months to ship in the following release. Once this cycles green I'll ping Marshall on the mailing list.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.34%. Comparing base (3205ae9) to head (06813c4).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1379   +/-   ##
=======================================
  Coverage   95.34%   95.34%           
=======================================
  Files         825      825           
  Lines       68160    68164    +4     
=======================================
+ Hits        64987    64991    +4     
  Misses       3173     3173           

Files with missing lines	Coverage Δ
include/boost/math/special_functions/ellint_2.hpp	100.00% <100.00%> (ø)
test/test_ellint_2.hpp	100.00% <100.00%> (ø)

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[jzmaddock]

Good idea!

[mborland]

Marshall approved. Merging.

[StephanTLavavej]

Thank you so much, this will help me pick it up in MSVC so much sooner! (I wish I had gotten around to reporting this upstream to you earlier, it was sitting on my plate for some time. If only all my upstream dependencies were as awesome as you!!)