Fix return value passing in pipe helper

addon/helpers/pipe.js

@@ -12,7 +12,7 @@ export function pipe(actions = []) {
         return curr(...args);
       }
 
-      return curr(acc, ...args);
+      return curr(acc);
     }, undefined);
   };
 }

tests/unit/helpers/pipe-test.js

@@ -11,9 +11,22 @@ function square(x) {
   return x * x;
 }
 
+function subtract(x, y) {
+  if (typeof y === 'undefined') {
+    throw new Error('2nd argument is undefined');
+  }
+  return x - y;
+}
+
 test('it pipes functions together', function(assert) {
   let piped = pipe([add, square, Math.sqrt]);
   let result = piped(2, 4);
 
   assert.equal(result, 6, 'it pipes functions together');
 });
+
+test('first function is variadic, rest are unary', function(assert) {
+  let piped = pipe([add, square, Math.sqrt, subtract]);
+
+  assert.throws(() => piped(2, 4), 'should throw an error');
+});