Correct ellint_2 for N * PI / 2 phi argument.

[jzmaddock]

Fixes #1377.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.34%. Comparing base (7bb5ceb) to head (c793e1c).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1378      +/-   ##
===========================================
- Coverage    95.34%   95.34%   -0.01%     
===========================================
  Files          825      825              
  Lines        68160    68164       +4     
===========================================
+ Hits         64987    64990       +3     
- Misses        3173     3174       +1     

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)

... and 1 file with indirect coverage changes

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