Convert Request, Response, CodeResponseType, TokenResponseType to ES6 classes

[menewman]

Summary

Chipping away at #212 by converting four more classes:

• Request
• Response
• CodeResponseType
• TokenResponseType

(Do note that TokenResponseType's constructor simply throws a not implemented error, though. So that conversion was easy. 😉)

Linked issue(s)

A few checkboxes of #212 (but not the whole thing).

Involved parts of the project

Specifically the classes noted above, which affect pretty much everything (since Request/Response are core parts of the interface). But, there are no functional changes.

Added tests?

I did not update the tests, but these files are already covered under the existing tests, which should continue to pass after this refactoring.

OAuth2 standard

N/A

Reproduction

N/A

[jorenvandeweyer]

@menewman I got some small comments, could you please update these?

@jankapunkt PR seems fine. Could you also review this so this one can be merged?

[jankapunkt]

There are some minor changes but I think at keast to has own property checks should be considered, even if the attack surface is rather superficial and imaginary