Fix silently failing specs

Two separate problems were hidden, despite a green suite:

- RSpec config problem
`geocoder_helper.rb` was breaking RSpec due to a bad configuration.
It made RSpec see feature specs examples as passing, while it ignores them completely.
After fixing the configuration (`before` instead of `around` for clearing `FakeGeocoder`),
few specs became red, due to real problems with the code, or only missing stubs for geocoding requests.

- Double "geocoding service" leak
This double was created in one example but leaks into another example.
rspec-mocks' double doesn't allow that, so we needed to introduce `spec/support/geocoder_service.rb`
that sets back `Place.geocoder_service` to it's original value after using a double in an example.
Nice ! Good idea.