feat(mime): Add Custom content-type for MultipartWriter

[getspooky]

It's good idea to specify custom headers:

const buf = new Buffer();
const mw = new MultipartWriter(buf);
await mw.writeField("foo", "foo");
await mw.writeField("bar", "bar");
const f = await Deno.open(path.join(testdataDir, "sample.txt"), {
    read: true,
 });
const header = new Headers();
header.set("deno", "deno");
await mw.writeFile("file", "sample.txt", f, header);
await mw.close();
f.close();

[ry]

This is what the "createPart" method is for?

[getspooky]

createPart is private method. i don't think there is a way to pass custom header ?
I think possible solution is to pass custom headers from createFormFile to createPart .

[ry]

@getspooky Have a look at the Go version of this module, from which this is based: https://golang.org/pkg/mime/multipart/#Writer.CreatePart
https://golang.org/pkg/mime/multipart/#Writer.WriteField
The signatures here should match those in Go. So it seems like createPart should be public.

[kt3k]

LGTM