Handle NaNs and Infs in gamma_inc

[andreasnoack]

These came up when testing JuliaStats/StatsFuns.jl#113.

Fixes #379

[codecov]

Codecov Report

Merging #380 (4043879) into master (51c5cd6) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   92.32%   92.40%   +0.08%     
==========================================
  Files          12       12              
  Lines        2789     2793       +4     
==========================================
+ Hits         2575     2581       +6     
+ Misses        214      212       -2     

Flag	Coverage Δ
unittests	92.40% <100.00%> (+0.08%)	⬆️

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Impacted Files	Coverage Δ
src/gamma_inc.jl	91.80% <100.00%> (+0.05%)	⬆️
src/beta_inc.jl	92.71% <0.00%> (+0.29%)	⬆️

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[devmotion]

Looks good to me. One additional suggestion would be to check the order of the branches, i.e., that e.g. gamma_inc(NaN, -1.0), gamma_inc(-1.0, NaN), and gamma_inc(1.0, -Inf) still throw an error and that gamma_inc(NaN, Inf) returns NaN.

[andreasnoack]

Sounds good. I've added the extra tests.