Add specs for Rex::OLE::Header

Adds specs for the Rex::OLE::Header class

Verification steps
-----------------------

- [ ] Be sure which travis is green
- [ ] Run the specs locally and ensure all the tests pass

```
$ rspec spec/lib/rex/ole/header_spec.rb
Rex::OLE::Header ..............................................................................

Finished in 0.08195 seconds
78 examples, 0 failures
```