Add basic support for parsing multipart/form-data fetch responses

[das7pad]

Hi!

For jenkinsci/jenkins#11177 we need basic support for multipart/form-data responses. I would like to contribute a simple implementation for it. It only handles the happy path well. Based on mozilla/rhino#1941, I take that a proper Fetch API implementation is planned for the long-term.

Testing:

• Copy and paste the contents of src/main/resources/org/htmlunit/javascript/polyfill/fetch/fetch.umd.js into the dev-tools of a browser
• Parse the sample multipart/form-data response from the tests

r=new window.WHATWGFetch.Response(
  "--0123456789\r\nContent-Disposition: form-data;name=test0\r\nContent-Type: text/plain\r\n\r\nHello1\nHello1\r\n--0123456789\r\nContent-Disposition: form-data;name=test1\r\nContent-Type: text/plain\r\n\r\nHello2\nHello2\r\n--0123456789--",
  { headers: { 'Content-Type': "multipart/form-data; boundary=0123456789"} }
);
f=await r.formData();
console.assert(f.get('test0') === 'Hello1\nHello1');
console.assert(f.get('test1') === 'Hello2\nHello2');
console.log(Object.fromEntries(Array.from(f.entries())));

It asserts that the two parts are properly parsed and prints them:

{test0: 'Hello1\nHello1', test1: 'Hello2\nHello2'}

[rbri]

@das7pad thanks for that. This is just in time and a great motivation. Have started some days ago with writing some fetch test to start a real impl (not that polyfill) for fetch. Will take your request her more as a motivation to finish that (hopefully with the 4.19.0 release.

Will have a look at this and maybe squezze it into 4.18.0.

[rbri]

Merged, adjusted, and some HtmlUnit fixes done. Your case passes now.
Thanks for this