[swift-corelibs-dev] NSURLRequest — missing attributes

[Daniel Eggert]

I created https://github.com/apple/swift-corelibs-foundation/pull/290
to add 4 attributes that were missing compared to Darwin Foundation.

I also re-worked how attributes are used / stored for the immutable / mutable version.

I didn't add any documentation to the 4 attributes, because I didn't know if we're allowed to copy over documentation from the Darwin Foundation header files. Can someone clarify?

/Daniel