Fix rspec for Rex::Exploitation::RopDb

Obviously I suck at rspec and failed to correctly handle the rspec going with #2467. This pull request tries to add the feedback from @limhoff-r7 and available on the pull request.

Probably review from @limhoff-r7 and @wchen-r7 is needed here before landing it.

Verification:

- [ ] Wait for the @limhoff-r7 and @wchen-r7's okey to it.
- [ ] run ```rspec spec/lib/rex/exploitation/ropdb_spec.rb```. Ensure the rspec runs ok:

```
$ rspec spec/lib/rex/exploitation/ropdb_spec.rb 

Rex::Exploitation::RopDb
  Class methods
    .initialize
      should initialize with a path of the ROP database ready
    .has_rop?
      should find the msvcrt ROP database
      should find the java ROP database
      should find the hxds ROP database
      should find the flash ROP database
      should return false when I supply an invalid database
    .select_rop
      should return msvcrt gadgets
      should return msvcrt gadgets for windows server 2003
      should return msvcrt gadgets with a new base
    .generate_rop_payload
      should generate my ROP payload
      should generate my ROP payload with my stack pivot
    .get_safe_size
      should return 0xfffffed0 (value does not need to be modified to avoid null bytes)
      should return 0xfffffeff (value is modified to avoid null bytes)
    .get_unsafe_size
      should return 0xfffffc00 (contains a null byte)

Finished in 0.1009 seconds
14 examples, 0 failures

```