util.iterators: join: Work even with only a single iterator in the chain

spec/util_iterators_spec.lua

@@ -10,6 +10,14 @@ describe("util.iterators", function ()
 			end
 			assert.same(output, expect);
 		end);
+		it("should work with only a single iterator", function ()
+			local expect = { "a", "b", "c" };
+			local output = {};
+			for x in iter.join(iter.values({"a", "b", "c"})) do
+				table.insert(output, x);
+			end
+			assert.same(output, expect);
+		end);
 	end);
 
 	describe("sorted_pairs", function ()

util/iterators.lua

@@ -240,7 +240,8 @@ function join_methods:prepend(f, s, var)
 end
 
 function it.join(f, s, var)
-	return setmetatable({ {f, s, var} }, join_mt);
+	local t = setmetatable({ {f, s, var} }, join_mt);
+	return t, { t, 1 };
 end
 
 return it;