Fix zeta sign flipping in some input regions (#420)

[quytelda]

As described in issue #420, zeta(s) actually returns $-\zeta(s)$ for some input regions. This patch fixes the issue and adds a few corresponding tests.

[codecov]

Codecov Report

Base: 91.87% // Head: 93.46% // Increases project coverage by +1.58% 🎉

Coverage data is based on head (b8ec371) compared to base (2cbb4ae).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
+ Coverage   91.87%   93.46%   +1.58%     
==========================================
  Files          14       14              
  Lines        2967     2908      -59     
==========================================
- Hits         2726     2718       -8     
+ Misses        241      190      -51     

Flag	Coverage Δ
unittests	93.46% <ø> (+1.58%)	⬆️

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Impacted Files	Coverage Δ
src/gamma.jl	94.52% <ø> (ø)
src/beta_inc.jl	91.16% <0.00%> (-1.20%)	⬇️
src/gamma_inc.jl	93.46% <0.00%> (+1.84%)	⬆️
src/expint.jl	94.34% <0.00%> (+5.04%)	⬆️
src/erf.jl	100.00% <0.00%> (+5.84%)	⬆️
src/sincosint.jl	100.00% <0.00%> (+34.54%)	⬆️

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