improve copy_with_eltype

[jverzani]

More work in #449

[codecov]

Codecov Report

Patch coverage: 81.81% and project coverage change: +0.05 🎉

Comparison is base (5df7fe9) 80.64% compared to head (f3a4107) 80.69%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
+ Coverage   80.64%   80.69%   +0.05%     
==========================================
  Files          25       25              
  Lines        3151     3150       -1     
==========================================
+ Hits         2541     2542       +1     
+ Misses        610      608       -2     

Impacted Files	Coverage Δ
src/polynomials/ImmutablePolynomial.jl	91.66% <75.00%> (+0.07%)	⬆️
src/common.jl	87.65% <83.33%> (+0.43%)	⬆️
src/polynomials/factored_polynomial.jl	86.45% <100.00%> (ø)

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