refactor(ajax): Move to simple Observable implementation. Fix errors.

[benlesh]

• Refactors ajax to use a simple Observable implementation.
• Moves code to a new ajax-specific folder.
• Adds a lot more documentation to classes and support classes.
• Differentiates between the configuration passed to ajax (AjaxConfig) and the request values used to make the HTTP request (AjaxRequest), as the latter has more required values.
• Ensures that no configuration values are mutated while making the request. (resolves Observable Ajax POST with wrong Content-Type after PUT #2801)
• Ensures there is at least one valid test for ajax.patch. The old one was sketchy.
• Adds better comments throughout the code.
• Adds better typing to ajax functions.
• Also ensures x-requested-with header is set properly
• Adds comments
• Code clean up
• Normalize headers to be lowercase, since they are case insensitive.
• Updates tests
• Remove superfluous checks in unsubscribe
• Ajax will now allow default behavior as defined in the specs:
  • https://xhr.spec.whatwg.org/#the-send()-method
  • https://fetch.spec.whatwg.org/#concept-bodyinit-extract
• if the body is set to: Blob, ArrayBuffer, any array buffer view (like a byte sequence, e.g. Uint8Array, etc), FormData, URLSearchParams, string, or ReadableStream, default handling is use. If the body is otherwise typeof "object", then it will be converted to JSON via JSON.stringify, and the Content-Type header will be set to application/json;charset=utf-8. All other types will emit an error. (resolves AJAX Content-Type Handling. #2837)
• Puts the behavior in-line with Axios, et al.

BREAKING CHANGE: For TypeScript users, AjaxRequest is no longer the type that should be explicitly used to create an ajax. It is now AjaxConfig, although the two types are compatible, only AjaxConfig has progressSubscriber and createXHR.

BREAKING CHANGE: ajax body serialization will now use default XHR behavior in all cases. If the body is a Blob, ArrayBuffer, any array buffer view (like a byte sequence, e.g. Uint8Array, etc), FormData, URLSearchParams, string, or ReadableStream, default handling is use. If the body is otherwise typeof "object", then it will be converted to JSON via JSON.stringify, and the Content-Type header will be set to application/json;charset=utf-8. All other types will emit an error.

BREAKING CHANGE: The Content-Type header passed to ajax configuration no longer has any effect on the serialization behavior of the AJAX request.

[benlesh]

As I'm going through the backlog of issues, I keep finding stuff related to ajax, so I've added a couple more commits to fix another (nasty) issue.

[cartant]

This also seems to need an API guardian update.

[benlesh]

The API guardian is killing me. It seems like there's an issue where you have to rebase, THEN update api_guardian before it actually works. This is the third PR where I've had issues. The thing it's complaining about is unrelated to the PR.