Fix #11 (#12)

* Add failing test

* Fix #11

Author: cmcaine

src/operators.jl

@@ -10,7 +10,10 @@ for op in (:+, :-, :&, :|, :xor, :(==))
             N = promote_type(T, S)
             ($op)(N(a), N(b))
         end
-        ($op)(a::S, b::Cenum{T}) where {T<:Integer,S<:Integer} = ($op)(b, a)
+        function ($op)(a::T, b::Cenum{S}) where {T<:Integer,S<:Integer}
+            N = promote_type(T, S)
+            ($op)(N(a), N(b))
+        end
     end
 end
 

test/runtests.jl

@@ -24,3 +24,6 @@ using Test
 @test Wed == 3 == suiyoubi
 @test Fri == 5
 @test Sat == 6
+
+# issue #11
+@test 0 - one == -1