`isequal` gives incorrect results for `NaN`

[AayushSabharwal]

Polynomials containing NaN don't obey the rules of isequal

julia> using DynamicPolynomials
julia> @polyvar p
julia> poly = p + NaN
julia> isequal(poly, poly)
false

julia> isequal(NaN, NaN)
true

This is especially problematic e.g. if polynomials are keys in a Dict:

julia> dict = Dict(poly => 2)
julia> poly2 = p + NaN
julia> dict[poly2]
ERROR: KeyError: key NaN + p not found

[AayushSabharwal]

I accidentally opened this here instead of DynamicPolynomials.

EDIT: JuliaAlgebra/DynamicPolynomials.jl#177