I discovered this bug while investigating issues with the updates process related to #1190#1292, and #814.
The `transportation_name` layer produces slightly different `tags` hstore values in the `osm_transportation_name_linestring` table during the initial import versus when running an update. As currently written, the import code produces null-value keys in the `tags` column, while the update code suppresses them. This PR removes that difference and makes the import code use same method that is currently used in the update code.
With a test case I've written, the import code produces a tags hstore that looks like this:
`"name"=>"OpenMapTiles Secondary 2", "name:de"=>NULL, "name:en"=>NULL, "name_int"=>"OpenMapTiles Secondary 2", "name:latin"=>"OpenMapTiles Secondary 2"`
...while the update code produces a tags hstore that looks like this:
`"name"=>"OpenMapTiles Secondary 2", "name_int"=>"OpenMapTiles Secondary 2", "name:latin"=>"OpenMapTiles Secondary 2"`
Note the missing NULL values.
This bug causes a small amount of space wastage after an update is run, because the update matching code detects the `tags` value as different, resulting in a duplicate copy of the tags value if that row is updated. This causes duplicate objects and breaks GROUP BY clauses that expect to group same-tagged features together. I've tested this by inspection of a generated mbtiles, database spot checks, and the unit test code included in this PR.
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