* Electron version: 1.6.3
* Operating system: Linux

### Expected behavior

When registering a `beforeunload` handler which cancels unloading, I would expect the handler to be called for every `BrowserWindow.reload()`.

### Actual behavior

On 1.6.3, the `beforeunload` handler is called for the first `reload` but not on subsequent calls. Even after removing the event handler, `reload` will not reload the window anymore.

### How to reproduce

```javascript
const cancelUnload = e => {
   console.log('cancelUnload called')
   e.returnValue = false
}

window.addEventListener('beforeunload', cancelUnload)

remote.getCurrentWindow().reload()  // [1] -> cancelUnload called
remote.getCurrentWindow().reload()  // [2] -> cancelUnload called

remote.getCurrentWindow().close()   // [3] -> cancelUnload called
remote.getCurrentWindow().close()   // [4] -> cancelUnload called

window.removeEventListener('beforeunload', cancelUnload)

remote.getCurrentWindow().reload()  // [5] -> reloads window

remote.getCurrentWindow().close()   // [6] -> closes window
```
I would expect `cancelUnload` to be printed to the console four times and the step at `[5]` to actually reload the window. This is works up to Electron 1.6.2.

On Electron 1.6.3 `cancelUnload` is not printed at step `[2]` (nor on any subsequent calls to reload). What's more, event after removing the `beforeunload` handler, `reload` does not work anymore: at step `[5]` the window does not reload (but closing the window at `[6]` works).
atom/browser/api/atom_api_web_contents.cc
bool b = false;
  if (options.Get("isGuest", &b) && b)
    type_ = WEB_VIEW;
  else if (options.Get("isBackgroundPage", &b) && b)
    type_ = BACKGROUND_PAGE;
  else if (options.Get("isBrowserView", &b) && b)
    type_ = BROWSER_VIEW;
  else if (options.Get("offscreen", &b) && b)
    type_ = OFF_SCREEN;
  
  options.Get("devTools", &enable_devtools_);
  
  std::string partition;
  mate::Handle<api::Session> session;
  if (options.Get("session", &session)) {
  } else if (options.Get("partition", &partition)) {
    session = Session::FromPartition(isolate, partition);
  } else {
    
    session = Session::FromPartition(isolate, "");
  }
  session_.Reset(isolate, session.ToV8());
  content::WebContents* web_contents;
  if (IsGuest()) {
    scoped_refptr<content::SiteInstance> site_instance =
        content::SiteInstance::CreateForURL(
            session->browser_context(), GURL("chrome-guest://fake-host"));
    content::WebContents::CreateParams params(
        session->browser_context(), site_instance);