Add specs for Rex::MIME

Also fixes a couple of bugs while parsing data. Even when it's not a (super) used feature (parsing data to initialize objects) worths to add some fixes I think.

Verification
----------

- [ ] Run rspec for the new specs and ensure there aren't failures

```
$ rspec spec/lib/rex/mime/
Rex::MIME::Part .............
Rex::MIME::Header .....................
Rex::MIME::Message ..............................................................
Rex::MIME::Encoding ....

Finished in 0.11766 seconds
100 examples, 0 failures
```

- [ ] Ensure travis is green before landing
 
mmm would love to listen more opinions about that. Honestly I feel like ```.new``` does more things than just ```#initialize```, but the only code I want (can) spec here is ```#initialize```. Because of that, I like this way of spec ```#initialize```. I don't think ```.send(:initialize``` is so weird. Honestly, I feel like spec'ing ```.new``` is more weird when indeed I would like to spec ```#initialize```. Any guidelines? 

(btw, I'm not an spec master, of course, and probably I'm forgetting things which probably make better to spec ```.new```. I won't resist too much to change :P. Just asking if there is a guidelines about spec ```.new``` vs ```#initialize``` and/or the reasons).