Incorrect GCD

[shashi]

julia> using DynamicPolynomials

julia> @polyvar x y
(x, y)

julia> gcd((-x + 1) * (y^2+1), -x+1)
-xy² + y² - x + 1

Found via JuliaSymbolics/Symbolics.jl#487

[blegat]

Should be fixed by #105