Add missing test case for vector:normalize() (#23)

Author: vil02

.spec/math/vector_spec.lua

@@ -52,6 +52,7 @@ describe("Vector", function()
 	it("normalization", function()
 		assert.same(vector.new({ 1, 2, 2 }) / 3, vector.new({ 1, 2, 2 }):normalize())
 		assert.same(vector.new({ 4, 13, 16 }) / 21, vector.new({ 4, 13, 16 }):normalize())
+		assert.same(vector.new({ 0, 0 }), vector.new({ 0, 0 }):normalize())
 	end)
 	it("angle between vectors", function()
 		assert_almost_equal(math.pi, vector.new({ 1, 1, 1 }):angle(vector.new({ -1, -1, -1 })))