Make BaseRequest (and subclasses) cloneable

[tvolkert]

For cases where you implement your own Client and override send(), it doesn't seem right to mutate the caller's request object.

It'd be better if BaseRequest were easily able to be copied. Not only would this allow you to keep from mutating the caller's object under their feet, but it would allow you to hide potentially sensitive information from the caller. e.g. in the use case that spawned this bug, the client is adding authentication headers to the request, and it's an explicit directive from the auth team that the scope of the OAuth token "leakage" be limited to auth libraries (that have undergone security review, etc.).

[tvolkert]

Hmm - looks like this may already exist on master and just not be released yet?

[nex3]

Yeah, this is one of the goals we're targeting for the refactoring @donny-dont is doing on master.

[nicowernli]

Hi guys, this is an old request but I still can't see any of this in the library and I still think it should be part of the lib.

I think that a method like copy or copyWith replacing certain values of the request will be really usefull, specially if you want to automatically modify the request on the send method.