Add specs for Rex::Encoder::NonAlpha

Verification
-----------

- [x] Be sure which travis is green
- [x] Run the specs locally and ensure all the test pass

```
$ rspec spec/lib/rex/encoder/nonalpha_spec.rb


WARNING: Cucumber-rails required outside of env.rb.  The rest of loading is being defered until env.rb is called.
  To avoid this warning, move 'gem cucumber-rails' under only group :test in your Gemfile
Rex::Encoder::NonAlpha ...................

Finished in 0.02455 seconds
19 examples, 0 failures

Randomized with seed 28544

Coverage report generated for RSpec to /Users/jvazquez/Projects/Code/metasploit-framework/coverage. 16396 / 92819 LOC (17.66%) covered.
```