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.
```
@kernelsmith @jhart-r7 I agree with @jhart-r7 on one thing: this code would benefit of code clean-up, lot of fixes, even refactoring, etc. As is the case with other code available in ```rex``` I'm reviewing while writing specs.

But the aim of these pull requests is to write specs for the **current behavior** and fix just things which **makes** framework / modules to not work. 

Improvements / refactorings / rewritings are not intended on these specs pull requests. But yeah, if you feel like some ```rex``` code needs to be fixed / refactored / rewritten in a urgent basis, feel free to close it and go for it! I won't be hurt!