Add more specs for Rex::OLE

Adds specs for:
* Rex::OLE::Header class
* Rex::OLE::DIFAT 
* Rex::OLE::MiniFat
* Rex::OLE::DirEntry

Verification steps
-----------------------

- [ ] Be sure which travis is green
- [ ] Run the specs locally one by one 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
```

```
$ rspec spec/lib/rex/ole/difat_spec.rb
Rex::OLE::DIFAT .....................................

Finished in 0.05008 seconds
37 examples, 0 failures

```

```
$ rspec spec/lib/rex/ole/minifat_spec.rb
Rex::OLE::MiniFAT ...........

Finished in 0.01647 seconds
11 examples, 0 failures

```

```
$ rspec spec/lib/rex/ole/direntry_spec.rb

Rex::OLE::DirEntry ...............................................

Finished in 0.05488 seconds
47 examples, 0 failures

```

* [ ] Also run all the Rex::OLE specs together and be sure they pass:

```
$ rspec spec/lib/rex/ole/*




Rex::OLE::Util ..........................................
Rex::OLE::MiniFAT ...........
Rex::OLE::DIFAT .....................................
Rex::OLE::Header ..............................................................................
Rex::OLE::CLSID ......
Rex::OLE::DirEntry ...............................................................

Finished in 0.3141 seconds
237 examples, 0 failures

```