Correct ibeta when it's an arcsine distribution.

[jzmaddock]

So we don't take the asin of values very close to 1.
Fixes: #1429.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.39%. Comparing base (5814eef) to head (0aa84df).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1432      +/-   ##
===========================================
- Coverage    95.40%   95.39%   -0.01%     
===========================================
  Files          829      829              
  Lines        69176    69186      +10     
===========================================
+ Hits         65997    66003       +6     
- Misses        3179     3183       +4     

Files with missing lines	Coverage Δ
include/boost/math/special_functions/beta.hpp	86.19% <100.00%> (+0.04%)	⬆️
test/test_students_t.cpp	99.58% <100.00%> (+<0.01%)	⬆️

... and 3 files with indirect coverage changes

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