Stat.histogram2d update

[Mattriks]

• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• enables Stat.histogram2d to work for the "single bin" case for the x and/or y axes.
• closes InexactError #860, closes Inexact error for histogram2d #965
• issue InexactError #860 provides a simple unit test

Example

# from #965        
pa = plot(x=randn(1000), y=ones(1000), Geom.histogram2d(xbincount=20))
pb = plot(x=randn(1000), y=ones(1000), Scale.y_discrete, Geom.histogram2d(xbincount=20))
hstack(pa,pb)

[tlnagy]

Thanks for squashing all these old bugs 🎉

[codecov-io]

Codecov Report

Merging #1237 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1237      +/-   ##
==========================================
+ Coverage   85.45%   85.67%   +0.22%     
==========================================
  Files          35       35              
  Lines        4083     4085       +2     
==========================================
+ Hits         3489     3500      +11     
+ Misses        594      585       -9

Impacted Files	Coverage Δ
src/bincount.jl	84.53% <100%> (+1.03%)	⬆️
src/statistics.jl	90.62% <100%> (+0.83%)	⬆️

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