delegate coefnames for wrapped models to model frame

[kleinschmidt]

This is a StatsBase API method that was missing.

fixes JuliaStats/GLM.jl#235

[codecov-io]

Codecov Report

Merging #60 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   93.85%   93.87%   +0.01%     
==========================================
  Files           6        6              
  Lines         358      359       +1     
==========================================
+ Hits          336      337       +1     
  Misses         22       22

Impacted Files	Coverage Δ
src/statsmodel.jl	86.84% <100%> (+0.35%)	⬆️

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[kleinschmidt]

The alternative way to implement this is to delegate it to the wrapped model itself, and leave it up to the model packages to do something sensible (like use the rownames of the coeftable). That might be slightly safer in cases where the model frame coefnames are not identical to the coeftable rownames, but I can't think of such a case off the top of my head...

[nalimilan]

Thanks! I agree that's the best approach, as the model does not currently have any notion of coefficient names anyway.