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
 
boundaries can be surrounded by ```"```, example:

```
Content-Type: multipart/mixed; boundary="_Part_12_3195573780_381739540"
```

Before it was failing to parse boundaries from content-types and because of that failing to create parts.

When allowing surrounding ```"``` noticed (from the RFC) which not every special character is allowed in a boundary. So added the allowed chars to the regexp. (Indeed spaces are allowed in the boundary, aren't allowed just at the end of the boundary, left that behavior as before (not allowing spaces in the boundary at all)).