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
 
You can do this without the join below:
```ruby
var, val = line.split(':', 2)
next unless val.nil?
```