Add specs for Rex::Exploitation::HeapLib

Adds specs for ```Rex::Exploitation::HeapLib```. While developing the specs noticed which the ```load_js``` behaviour isn't the expected if ```opts[:newobfu]``` is ```true```. This pull request also fixes it.

Verification
--------

- [x] Use rspec to check new specs

```
$ rspec spec/lib/rex/exploitation/heaplib_spec.rb
Rex::Exploitation::HeapLib .......

Finished in 0.1104 seconds
7 examples, 0 failures

Randomized with seed 35759

Coverage report generated for RSpec to /metasploit-framework/coverage. 16179 / 91892 LOC (17.61%) covered.
```

- [x] Be sure which Travis is green