datafusion-proto deserialize with Substring(str [from int] [for int]) fails

[r4ntix]

Describe the bug
Serialize and Deserialize with Substring(str [from int] [for int]):

// substr(string, position, count)
let test_expr_with_count = Expr::ScalarFunction {
    fun: Substr,
    args: vec![col("col"), lit(1_i64), lit(1_i64)],
};

let ctx = SessionContext::new();
roundtrip_expr_test(test_expr_with_count, ctx);

Failed:

assertion failed: `(left == right)`
  left: `"substr(col, Int64(1), Int64(1))"`,
  right: `"substr(col, Int64(1))"`

This problem was found in arrow-ballista's benchmarks test: apache/datafusion-ballista#375

To Reproduce
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Expected behavior
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Additional context
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