specs for Rex::Encoder::XDR

Verification
-----------

- [ ] Be sure which travis is green
- [ ] Run the specs locally and ensure all the test pass

```
$ rspec spec/lib/rex/encoder/xdr_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::XDR ....................................

Finished in 0.04763 seconds
36 examples, 0 failures

Randomized with seed 5417

Coverage report generated for RSpec to /Users/jvazquez/Projects/Code/metasploit-framework/coverage. 16410 / 92792 LOC (17.68%) covered.
```
I wasn't suggesting that you add input validation.  My point was that you are unit testing the way this is implemented rather than what it is supposed to do.  I believe the implementation is wrong.