support reserved words as fn name

Because `thenify-all` throws if module that thenifing exports/have methods named with reserved word.

fixture.js
```js
module.exports = function Fixture() {}

Fixture.delete = function(cb) {
  cb(1,2,3)
};
Fixture.throw = function _throw(cb) {
  cb('a', 'b')
};
```
then
```js
var fixture = require('fixture');
var thenifyAll = require('thenify-all');

fixture = thenifyAll(fixture, fixture)
```
will throw with error
```js
(function delete() {
          ^^^^^^
SyntaxError: Unexpected token delete
```

Also need to be tested in thenify-all, I'll come with PR there soon.