Fix overflow in mean(::AbstractRange)

[jishnub]

Fixes #120, now

julia> mean(Int8(123):Int8(123))
123.0

[codecov-commenter]

Codecov Report

Merging #122 (9eb9a8e) into master (0588f2c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #122   +/-   ##
=======================================
  Coverage   96.94%   96.95%           
=======================================
  Files           1        1           
  Lines         426      427    +1     
=======================================
+ Hits          413      414    +1     
  Misses         13       13           

Impacted Files	Coverage Δ
src/Statistics.jl	96.95% <100.00%> (+<0.01%)	⬆️

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[jishnub]

Seems to be a duplicate of #115