Fix broken spec for Rex::Text.randomize_space

cc @OJ

**Problem description:**

#3170 -> https://github.com/rapid7/metasploit-framework/pull/3121#issuecomment-39354215

**Verification steps:**

- [x] Check out the topic branch
- [x] Run ```while rspec spec/lib/rex/text_spec.rb; do continue; done```
- [x] See that there are no failures
- [x] ```^C``` when you get annoyed at the lack of failures

**Sample run:**

```
wvu@kharak:~/metasploit-framework:beug/randomize_space$ while rspec spec/lib/rex/text_spec.rb; do continue; done
Rex::Text ...............

Finished in 0.00734 seconds
15 examples, 0 failures
[snip]
Rex::Text ...............

Finished in 0.00726 seconds
15 examples, 0 failures
^C
Exiting... Interrupt again to exit immediately.

Finished in 0.00004 seconds
0 examples, 0 failures
wvu@kharak:~/metasploit-framework:beug/randomize_space$ 
```