Stat.histogram update

[Mattriks]

• I've 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:

• fixes histogram(density = true) and x_discrete(labels = ...) incompatible ? #1163, fixes Histogram binning issue? #1235

Examples

# from #1163
x_cont = [1,1,1,1,1,2,2,2,2]
x_disc = string.(x_cont)
p1 = plot(x=x_cont, Geom.histogram(density=true))
p2 = plot(x=x_disc, Geom.histogram(density=true))
hstack(p1, p2)

# from #1235 
df = DataFrame(Num_digits=[1,1,1,1,1,2,2,2,2,2,2],
    Primality=[false,true,true,false,false,false,true,false,false,false,true])
p3 = plot(df, x=:Num_digits, Geom.histogram(position=:stack), Scale.x_discrete, color=:Primality)
p4 = plot(df, x=:Num_digits, Geom.histogram(position=:dodge), Scale.x_discrete, color=:Primality)
hstack(p3, p4)

[codecov-io]

Codecov Report

Merging #1236 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1236      +/-   ##
==========================================
+ Coverage   85.37%   85.45%   +0.07%     
==========================================
  Files          35       35              
  Lines        4096     4083      -13     
==========================================
- Hits         3497     3489       -8     
+ Misses        599      594       -5

Impacted Files	Coverage Δ
src/statistics.jl	89.78% <100%> (+0.37%)	⬆️

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