[Feature] request.postData to return json string instead of urlsearchparams

[aesyondu]

I noticed that when using route to assert postData:

page.route("some-route", (route, request) => {
  expect(request.postData()).toMatchObject({ foo: "bar" })
})

that it returns a string of search params foo=bar&baz=bum.

Does it make sense to make it return a json string instead?

Reference commit: aesyondu@1b3929d, with test case and sample naive implementation

Given:
A form with input type text, input type number, and input type submit

await page.setContent(`<form method='POST' action='/post'><input type='text' name='foo' value='bar'><input type='number' name='baz' value='123'><input type='submit'></form>`);
await page.click('input[type=submit]');
expect(request).toBeTruthy();
expect(request.postData()).toBe('{"foo":"bar","baz":"123"}');

When:
Form data is submitted via POST method.

Then:
request.postData() should return a json string: {"foo":"bar","baz":"3"}

[pavelfeldman]

According to the spec:

As part of a POST request, an arbitrary amount of data of any type can be sent to the server in the body of the request message. A header field in the POST request usually indicates the message body's Internet media type.

We could add postDataJSON though.