Add specs for the Typo3 mixin

This pull requests adds specs for the the ```Msf::HTTP::Typo3```. In addition, when writing the specs, noticed a bug on ```typo3_backend_login```, when trying to login against a non typo3 app. Fixed too.

This pull request has several motivations:

* Training myself on specs, so feedback is welcome.
* More specs for the framework.
* Hopefully be useful as guide to #3540, since that pull request needs similar specs.

Verification
--------

- [ ] run the specs, verify everything passes

```
$ rspec spec/lib/msf/http/typo3.rb
Msf::HTTP::Typo3 .........

Finished in 1.11 seconds
9 examples, 0 failures
Coverage report generated for RSpec to /Users/jvazquez/Projects/Code/metasploit-framework/coverage. 15387 / 89775 LOC (17.14%) covered.
```