feat(XMLHttpRequest): use proxies to implement request interception

[kettanaito]

• Fixes XMLHttpRequest: readystatechange listener is called twice for unhandled requests #103
• Fixes XMLHttpRequest: throw when setting "responseType" on invalid request state #311
• Closes fix(XMLHttpRequest): forward "responseType" and "withCredentials" to the bypassed request #310 (verified in 4724ae8)
• Fixes XMLHttpRequest: "responseType" is not being honored when making non-mocked requests #309

It should be safe migrating to Proxies due to their wide browser support.

Changes

• Uses ES6 Proxies to spy on globalThis.XMLHttpRequest.
• Proxies request.headers changes of the Fetch API representation of an XMLHttpRequest to update the underlying XHR instance correctly (in the case the "request" listener has modified the request headers).
• Fixes the issue where a readystatechange event has been fired twice for XMLHttpRequest: once from the XHR instance itself, and the second time from the underlying http implementation from JSDOM.
• Sets credentials to same-origin (previously omit) when the withCredentials property of XMLHttpRequest is set to false or not set.
• Deleting a request header in the "request" event listener will now raise a warning for XMLHttpRequest. XHR headers cannot be deleted per specification.
• Spec-compliance: We now throw InvalidStateError when reading the request body improperly (e.g. using responseText when the responseType is not an empty string or "text"). This also helps to improve some of our tests as they were reading the response body of XMLHttpRequest incorrectly.

Roadmap

• Add internal logger to XMLHttpRequestController
• Emit the "response" event upon bypassed responses. There's also no test for this, apparently.
• Remove the previous implementation (XMLHttpRequestOverride).
• Consider if setting the X-Request-Id on the bypassed requests is still needed:
  

  interceptors/src/interceptors/XMLHttpRequest/XMLHttpRequestOverride.ts

  Lines 472 to 477 in d73624b

  	* @note Set the intercepted request ID on the original request
  	* so that if it triggers any other interceptors, they don't attempt
  	* to process it once again. This happens when bypassing XMLHttpRequest
  	* because it's polyfilled with "http.ClientRequest" in JSDOM.
  	*/
  	originalRequest.setRequestHeader('X-Request-Id', requestId)

[kettanaito]

Released: v0.20.0 🎉

This has been released in v0.20.0!

• 📄 Release notes
• 📦 npm package

Make sure to always update to the latest version (npm i @mswjs/interceptors@latest) to get the newest features and bug fixes.

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