MultipartReader creates broken files

[marcosc90]

MultipartReader truncates big files.

version:

deno 0.41.0
v8 8.2.308
typescript 3.8.3

To reproduce run the following snippet:

import { serve } from "https://deno.land/std/http/server.ts";
import { MultipartReader } from "https://deno.land/std/mime/multipart.ts";

function getHeaderValueParams(value) {
  const params = new Map();
  // Forced to do so for some Map constructor param mismatch
  value
    .split(";")
    .slice(1)
    .map((s) => s.trim().split("="))
    .filter((arr) => arr.length > 1)
    .map(([k, v]) => [k, v.replace(/^"([^"]*)"$/, "$1")])
    .forEach(([k, v]) => params.set(k, v));
  return params;
}

const s = serve({ port: 8000 });
for await (const req of s) {
    const params = getHeaderValueParams(req.headers.get("content-type"))
    
    const reader = new MultipartReader(req.body, params.get('boundary'));

    const form = await reader.readForm(1 << 20)
    console.log(form.file('file'))
    req.respond({ body: 'deno' });
}

Any file with a size bigger than 11534332 bytes will get truncated.

truncate -s 12m 12m.file

curl --location --request POST 'http://localhost:8000/' \
--form 'deno=land' \
--form 'file=@/home/marcos/12mb.file'
# change file path