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 think it's more related to unexpected input (nil in this case). Honestly I'm not trying to add input validation to every framework method in these specs pull requests, just describe the current behavior (fix if something is hardly damaged, of course). 

I would like to avoid trying to add input validation on every method not doing it currently on framework, otherwise specs are going to be written every moooooooore slowly (I would like to insist in the mooooooooore word :P)