Geocode tests

_Please discard my previous pull request on this subject. This pull request supersedes it._

* extracted the checks for latitude and longitude in `run_geocode?` into a separate `not_geocoded?` method, which allows for easy reading and thus is more self-documenting.
* refactored booleans in `run_geocode?` into a single, more succinct boolean 
* wrote rspec unit tests for `not_geocoded?`
* refactored rspec unit tests for `run_geocode` to more carefully navigate all the possible paths among the booleans