Add support for URLRequest's httpBodyStream

[alexzagu]

Snapshotting an URLRequest instance after it has been processed by the URL Loading System (by subclassing URLProtocol) doesn't work properly at the moment, since the framework has support for httpBody but not for httpBodyStream.

While processing the request, the URL Loading System places the data that was in the httpBody into the httpBodyStream before sending it to any subclass of URLProtocol to be fulfilled. That's why, if you're using a spy subclass to catch requests in tests, snapshotting won't reveal the data inside the request as of now.

[grdsdev]

I'm also in need of that, can you review this @mbrandonw or @stephencelis? I see linked PR has merge conflicts, I can fix that if you're willing to merge it.

I'm currently running a fork because of that https://github.com/grdsdev/swift-snapshot-testing

[mbrandonw]

Hi @grdsdev, we do not plan on merging that PR at this time. However, instead of depending on a fork, why don't you simply extract that snapshot strategy out of the library and make any changes you want to it? The strategy does not use any internals of the library, and so you should be able to maintain it yourself without involving a fork.

[grdsdev]

Thanks, @mbrandonw I will do that.

Just out of curiosity, why wouldn't this get merged at this time? Is it an issue with implementation?