Full implementation of xhr.responseType, rigorous test and a perf imp…

[pringseth]

…rovement.
Implemented section 3.6, subsections 8,9,10,11 of https://xhr.spec.whatwg.org/#the-response-attribute.
The test: test-response-type.js excerises it well.
Basically async download looks like this:

    xhr.open("GET", url, true);

    xhr.responseType =  responseType;

    xhr.onloadend = () => {
      if (xhr.status >= 200 && xhr.status < 300)
      {
        switch (responseType)
        {
          case "":
          case "text":
            resolve(xhr.responseText);
            break;
          case "document":
            resolve(xhr.responseXML);
            break;
          default:
            resolve(xhr.response);
            break;
        }
      }
      else
      {
        const errorTxt = `${xhr.status}: ${xhr.statusText}`;
        reject(errorTxt);
      }
    };

    xhr.send();

In order to use xhr.responseType="document" the npm package "@xmldom/xmldom" needs to be installed.
There's also been a perf improvement (new test: test-perf.js ):

As part of the implementation, there has been a major perf improvement.
On a 3 year old i9-10900K, a local download using this changelist of a 400MB Float32Array 
from node 'http' module takes about 300ms. However, using xmlhttprequest-ssl v2.1.1 it takes 372734ms.
That's over 1200x faster.

[mjwwit]

Thanks for this! Looking at the scope of the changes I think I'll need some time to review it.

[mjwwit]

This test file actually contained one extra case; requesting binary data without setting xhr.responseType = 'arraybuffer'. This was previously covered on lines 36-48:

  xhr = new XMLHttpRequest();
  isSync = false;

  xhr.onreadystatechange = function () {
    if (xhr.readyState === 4) {
      assert.equal(xhr.response.toString(), 'Hello world!');
      assert.equal(xhr.getResponseHeader('content-type'), 'application/octet-stream')
      isSync = true;
    }
  }
  
  xhr.open("GET", "http://localhost:8888/binary", false);
  xhr.send();

When applying this test case to your changes in lib/XMLHttpRequest.js it fails. When checking browser behavior though, this case should work:

    var xhr = new XMLHttpRequest();

    xhr.onreadystatechange = function() {
      if (this.readyState == 4) {
        if ("Hello world!" !== this.response.toString()) {
            throw new Error(`Text does not match! ${this.response.toString()}`);
        }
      }
    };

    var url = "http://localhost:8888/binary";
    xhr.open("GET", url);
    xhr.send();

[mjwwit]

Do you think this warrants the addition of an optional peerDependency on @xmldom/xmldom?