Add rspec for Browser Autopwn options

rspec for browser autopwn options, that way we can make sure they're never lost. A lost key may cause lots of broken modules.

Verification steps:

- [x] Enter this command: ```rspec spec/lib/msf/core/exploit/browser_autopwn_spec.rb```
- [x] Confirm the test case passes the test like the following example:

```
$ rspec spec/lib/msf/core/exploit/browser_autopwn_spec.rb
Msf::Exploit::Remote::BrowserAutopwn::AutopwnClassMethods .

Finished in 1.03 seconds
1 example, 0 failures
```