Add zip code lookup with specs and VCR cassettes

[Gilmoursa]

This supersedes #28 with the missing test coverage added.

Changes

lib/open_weather/api.rb — adds .zip to ClassMethods, making it available on Current, Forecast, ForecastDaily, and History:

# Zip code format: '33704,US'
# Usage: OpenWeather::Current.zip('33704,US')
def zip(zip_code, options = {})
  new(options.merge(zip: zip_code)).retrieve
end

lib/open_weather/base.rb — adds :zip to valid_options so the parameter isn't stripped before the API request is sent.

spec/open_weather/api_spec.rb — adds a .zip context with two cases matching the style of every other method in the suite:

• valid zip (33704,US) → expects cod: 200
• invalid zip (00000,US) → expects cod: '404'

spec/fixtures/cassettes/api/ — adds current_zip_valid.yml and current_zip_invalid.yml VCR cassettes in the existing format.

README.md — moves the zip example into the Current API section (where the other single-location lookups live) and fixes a pre-existing "Doucumentation" typo.

Test plan

• bundle exec rspec spec/open_weather/api_spec.rb — all existing tests pass, two new .zip tests pass

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