fix: take schema from reader instead of builder

[Droeftoeter]

When fetching a subset of columns the full schema was returned, now it returns the projected schema instead.

Fixes #810

[github-actions]

Asset	Size	Compressed Size
report_pkg/async_full/parquet_wasm_bg.wasm	6.7MB	1.3MB
report_pkg/slim/parquet_wasm_bg.wasm	4.9MB	678KB
report_pkg/sync/parquet_wasm_bg.wasm	6.2MB	1.2MB

[kylebarron]

Thanks! Would you be able to add a JS test that failed before this change but passes after it? I figure just passing a column projection into a read function should test this code path