Add more rspec coverage for BrowserExploitServer

This PR makes sure all the methods in BrowserExploitServer are covered.

Verification steps:
- [ ] Enter this command: ```rspec spec/lib/msf/core/exploit/remote/browser_exploit_server_spec.rb```
- [ ] Confirm all 33 test cases pass like the following example:

```
$ rspec spec/lib/msf/core/exploit/remote/browser_exploit_server_spec.rb 
Msf::Exploit::Remote::BrowserExploitServer .................................

Finished in 4.96 seconds
33 examples, 0 failures
```