findfirst doesn't work for infranges with negative step sizes

[DanielVandH]

julia> using InfiniteArrays

julia> findfirst(==(9), 10:-1:-∞)

julia> findfirst(==(11), 10:-1:-∞)
0

julia> findfirst(==(21), 10:-1:-∞)
-10

julia> (10:-1:-∞)[-10]
ERROR: BoundsError: attempt to access ℵ₀-element InfiniteArrays.InfStepRange{Int64, Int64} with indices OneToInf() at index [-10]
Stacktrace:
 [1] throw_boundserror(A::InfiniteArrays.InfStepRange{Int64, Int64}, I::Int64)
   @ Base .\abstractarray.jl:737
 [2] getindex(v::InfiniteArrays.InfStepRange{Int64, Int64}, i::Int64)
   @ InfiniteArrays C:\Users\User\.julia\packages\InfiniteArrays\heAfT\src\infrange.jl:213
 [3] top-level scope
   @ REPL[18]:1