Fix ambiguous convert

[omus]

julia> using Infinity
[ Info: Precompiling Infinity [a303e19e-6eb4-11e9-3b09-cd9505f79100]

julia> convert(Infinite, InfExtended{Int64}(∞))
ERROR: MethodError: convert(::Type{Infinite}, ::InfExtended{Int64}) is ambiguous. Candidates:
  convert(::Type{Infinite}, x::Real) in Infinity at /Users/omus/.julia/dev/Infinity/src/conversion.jl:15
  convert(::Type{T}, x::InfExtended{S}) where {T<:Real, S<:Real} in Infinity at /Users/omus/.julia/dev/Infinity/src/conversion.jl:14
Possible fix, define
  convert(::Type{Infinite}, ::InfExtended{S<:Real})
Stacktrace:
 [1] top-level scope at REPL[2]:1

[nickrobinson251]

lgtm

[nickrobinson251]

we may as well test we didn't forget the signbit?

Suggested change

	@test convert(Infinite, InfExtended{Int}(∞)) === ∞
	@test convert(Infinite, InfExtended{Int}(∞)) === ∞
	@test convert(Infinite, InfExtended{Int}(-∞)) === -∞

[cjdoris]

Thanks!