Fix bug with optional parameter promise functions

Previously, we didn't remove the callback parameter from the
arguments when calling a promise function, this confused functions
with optional parameters, since the last parameter would be the
callback function.

index.js

@@ -20,6 +20,10 @@ exports.fromPromise = function (fn) {
   return Object.defineProperty(function () {
     const cb = arguments[arguments.length - 1]
     if (typeof cb !== 'function') return fn.apply(this, arguments)
-    else fn.apply(this, arguments).then(r => cb(null, r), cb)
+    else {
+      delete arguments[arguments.length - 1]
+      arguments.length--
+      fn.apply(this, arguments).then(r => cb(null, r), cb)
+    }
   }, 'name', { value: fn.name })
 }

test/from-promise.js

@@ -37,6 +37,29 @@ test('promise function works with promises', t => {
   .catch(t.end)
 })
 
+test('promise function optional param works with callbacks', t => {
+  t.plan(4)
+  fn.call({a: 'a'}, 1, (err, arr) => {
+    t.ifError(err, 'should not error')
+    t.is(arr[0].a, 'a')
+    t.is(arr[1], 1)
+    t.is(arr[2], undefined)
+    t.end()
+  })
+})
+
+test('promise function optional param works with promises', t => {
+  t.plan(3)
+  fn.call({a: 'a'}, 1)
+  .then(arr => {
+    t.is(arr[0].a, 'a')
+    t.is(arr[1], 1)
+    t.is(arr[2], undefined)
+    t.end()
+  })
+  .catch(t.end)
+})
+
 test('promise function error works with callbacks', t => {
   t.plan(2)
   errFn(err => {