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
 
Since you're doing this in each spec within this `describe`, you can just change the `subject`, with something like this:
```ruby
subject(:added_part) do
  described_class.new.add_part(*args)
end
let(:args) { [] }
```
And then for the couple of specs that need different arguments,
```ruby
context "with 'binary' Content-Type" do
  let(:args) { ['', 'application/pdf', 'binary'] }
  it ...
end
```