Add missing methods for RationalPoly

[ahumenberger]

Fixes JuliaAlgebra/DynamicPolynomials.jl#49

[codecov-io]

Codecov Report

Merging #117 into master will increase coverage by 0.05%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
+ Coverage   91.07%   91.13%   +0.05%     
==========================================
  Files          18       18              
  Lines         605      609       +4     
==========================================
+ Hits          551      555       +4     
  Misses         54       54

Impacted Files	Coverage Δ
src/operators.jl	97.1% <0%> (-2.9%)	⬇️
src/rational.jl	97.36% <100%> (+5.7%)	⬆️

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[coveralls]

Coverage increased (+0.7%) to 91.133% when pulling 9919445 on ahumenberger:master into fe0d7ca on JuliaAlgebra:master.

[blegat]

Thanks!