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

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.40%. Comparing base (5814eef) to head (c3656f9).

Files with missing lines	Patch %	Lines
include/boost/math/special_functions/beta.hpp	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1432   +/-   ##
========================================
  Coverage    95.40%   95.40%           
========================================
  Files          829      829           
  Lines        69176    69180    +4     
========================================
+ Hits         65997    66003    +6     
+ Misses        3179     3177    -2     

Files with missing lines	Coverage Δ
test/test_students_t.cpp	99.58% <100.00%> (+<0.01%)	⬆️
include/boost/math/special_functions/beta.hpp	86.07% <75.00%> (-0.09%)	⬇️

... and 1 file with indirect coverage changes

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5814eef...c3656f9. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.