Fix quantiles of degenerate beta distributions

[devmotion]

Fix #154 by handling degenerate beta distributions in quantile functions.

To make it clearer and more consistent I also added cases for the degenerate beta distribution with zero beta parameter to the other functions; currently we rely on how SpecialFunctions handles these degenerate cases.

[codecov-commenter]

Codecov Report

Base: 61.58% // Head: 62.79% // Increases project coverage by +1.20% 🎉

Coverage data is based on head (86d60fc) compared to base (7782629).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 86d60fc differs from pull request most recent head e22f78a. Consider uploading reports for the commit e22f78a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
+ Coverage   61.58%   62.79%   +1.20%     
==========================================
  Files          14       14              
  Lines         617      637      +20     
==========================================
+ Hits          380      400      +20     
  Misses        237      237              

Impacted Files	Coverage Δ
src/distrs/beta.jl	100.00% <100.00%> (ø)

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