Gracefully handle case where Feature is passed to FeatureIterator

[frewsxcv]

See https://github.com/georust/geojson/compare/frewsxcv-test-test?expand=1

This test fails with:

called `Result::unwrap()` on an `Err` value: Custom { kind: InvalidData, error: Error("invalid type: floating point `1`, expected a map", line: 1, column: 4) }

[frewsxcv]

This should either return a better error, or we should gracefully handle cases where the user passes in something that isn't a feature collection