PR #2421 More descriptive rspec

This PR pretty much completely changes up @FireFart's original rspec tests submitted on https://github.com/rapid7/metasploit-framework/pull/2421:

  * Defines the test HTTP headers as HEREDOCs for readability.
  * DRY the tests with a common test helper, cookie_sanity_check
  * Sets the context to just #get_cookies
  * Describes the test scenarios instead of just numbering them
  * Uses *array syntax, and a shuffle, in order to test expected result completeness.

As far as I can see, I haven't actually changed the meaning, but have made the results useful for when/if they ever break down.