Fix geojson_source_to_csv crash on non-uniform feature properties

[iandees]

Summary

• geojson_source_to_csv() picked its output CSV columns from the first feature's properties only, then crashed with ValueError: dict contains fields not in fieldnames as soon as a later feature had a property key the first feature didn't have.
• Whether it crashed was order-dependent on which feature came first in the file.
• Fixed by making a first pass over the file to collect the union of every feature's property keys (in first-seen order), then a second pass to write rows against that fixed set of fieldnames. Missing keys on a given feature fall back to DictWriter's default restval=''.

Fixes #106

Test plan

• Added test_geojson_source_to_csv_non_uniform_properties, reproducing the crash with two features that have different property keys
• docker run --rm -v "$(pwd)":/usr/local/src/batch-machine -w /usr/local/src/batch-machine batch-machine-test:latest python3 -m unittest openaddr.tests.conform -v — all 65 tests pass