* Electron version: 1.7.4
* Operating system: macOS Sierra

### Expected behavior

Load the PDF file correctly.

### Actual behavior

When I try to load something like http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1118&context=podimproveacad, the '&' symbol seems to interrupt the PDF Viewer to load the file.
![](http://i.imgur.com/RDSeY2u.png)

You can see that part of query string behind '&' get chopped, and I believe this is the reason why the PDF Viewer failed to load.

### How to reproduce

Clone the reproducible project at https://github.com/qazbnm456/electron-quick-start.
```
$ yarn # install packages
$ yarn run start # start the electron
```
atom/browser/atom_resource_dispatcher_host_delegate.cc
#include "atom/browser/atom_resource_dispatcher_host_delegate.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/login_handler.h"
#include "atom/browser/web_contents_permission_helper.h"
#include "atom/browser/web_contents_preferences.h"
#include "atom/common/atom_constants.h"
#include "atom/common/platform_util.h"
#include "base/strings/stringprintf.h"
#include "base/strings/utf_string_conversions.h"
#include "content/public/browser/browser_thread.h"
#include "content/public/browser/download_manager.h"
#include "content/public/browser/stream_info.h"
#include "net/base/escape.h"
#include "net/ssl/client_cert_store.h"
#include "net/url_request/url_request.h"
#include "url/gurl.h"
#if defined(USE_NSS_CERTS)
#include "net/ssl/client_cert_store_nss.h"
#elif defined(OS_WIN)
#include "net/ssl/client_cert_store_win.h"
#elif defined(OS_MACOSX)
#include "net/ssl/client_cert_store_mac.h"
#endif
using content::BrowserThread;
namespace atom {
namespace {
void OnOpenExternal(const GURL& escaped_url,