import/export fitted

[kleinschmidt]

fitted is defined in StatsBase.jl, and a method is defined in GLM.jl, but
fitted is not imported, and it's also not exported (as far as I can tell).
This PR just imports/exports fitted and adds one test to check that the method
resolves correctly and produces the same value as predict(::LinPredModel) (as
defined).

[codecov-io]

Codecov Report

Merging #389 (89b640c) into master (3234f5e) will decrease coverage by 0.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
- Coverage   79.56%   78.68%   -0.88%     
==========================================
  Files           7        7              
  Lines         685      685              
==========================================
- Hits          545      539       -6     
- Misses        140      146       +6     

Impacted Files	Coverage Δ
src/GLM.jl	100.00% <ø> (ø)
src/glmfit.jl	74.89% <0.00%> (-2.47%)	⬇️

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