add sum(count) case in PushDownProjection #49

Job	Run time
cargo test (mac)	4m 10s
Verify Vendored Code	1m 3s
Check cargo fmt	35s
cargo check	3m 57s
cargo test (win64)	9m 24s
cargo test hash collisions (amd64)	0s
clippy	0s
check Cargo.toml formatting	0s
check configs.md is up-to-date	0s
cargo doc	0s
cargo test (amd64)	0s
cargo test datafusion-cli (amd64)	0s
cargo test doc (amd64)	0s
cargo examples (amd64)	0s
Run sqllogictest with Postgres runner	0s
cargo test pyarrow (amd64)	0s
verify benchmark results (amd64)	0s
	19m 9s

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