fix(csv/csv_stringify_stream): output headers based on columns option

[uki00a]

This PR changes CsvStringifyStream to output headers when the columns option is given. This change is intended to make it easier to output header columns when streaming objects.

Example

// test.js
import { CsvStringifyStream } from "./csv/csv_stringify_stream.ts";
import { readableStreamFromIterable } from "./streams/readable_stream_from_iterable.ts";

const file = await Deno.open("data.csv", { create: true, write: true });
const readable = readableStreamFromIterable([
  { id: 1, name: "one" },
  { id: 2, name: "two" },
  { id: 3, name: "three" },
]);

await readable
  .pipeThrough(new CsvStringifyStream({ columns: ["id", "name"] }))
  .pipeThrough(new TextEncoderStream())
  .pipeTo(file.writable);

Before:

$ deno run --allow-write main.js
$ cat data.csv
1,one
2,two
3,three

After:

$ deno run --allow-write main.js
$ cat data.csv
id,name
1,one
2,two
3,three

[kt3k]

Makes sense. LGTM.