Fixes nil error in BrowserExploitServer

RM #8762

I also removed the synchronous AJAX POST of the gathered metadata. An extra request here adds some unnecessary overhead which is annoying on mobile connections. Instead I do an actual form POST, which the plugin immediately redirects to the exploit GET URL.

Verification:
- [ ] Run any module that uses BES, visit it in your (applicable) browser
- [ ] the module should serve the exploit
- [ ] refresh the exploit page in your browser
- [ ] you should not receive the exploit again
- [ ] `rerun` the module, and refresh it in your browser
- [ ] the module should serve the exploit again
- [ ] refresh the exploit page in your browser
- [ ] you should not receive the exploit again