Add specs for Rex::Encoder::Alpha2 encoders

Verification
-----------

- [ ] Be sure which travis is green
- [ ] Run the specs locally and ensure all the tests pass

```
$ rspec spec/lib/rex/encoder/alpha2/
Rex::Encoder::Alpha2::UnicodeMixed ..............
Rex::Encoder::Alpha2::UnicodeUpper ...............
Rex::Encoder::Alpha2::Generic ...........
Rex::Encoder::Alpha2::AlphaUpper ...............
Rex::Encoder::Alpha2::AlphaMixed ..............

Finished in 0.09253 seconds
69 examples, 0 failures


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