Swift 3 API Parity: Use URLRequest rather than NSURLRequest in APIs

[seabaylea]

A number of the URLSession APIs should now take URLRequest rather than NSURLRequest or NSMutableURLRequest as parameters. Examples of this include:

open func dataTask(with request: URLRequest) -> URLSessionDataTask
open func uploadTask(with request: URLRequest, from bodyData: Data) -> URLSessionUploadTask
open func downloadTask(with request: URLRequest) -> URLSessionDownloadTask

This PR switches over to using URLRequest rather than the existing use of NSURLRequest.

I've additionally added tests for URLRequest in addition to NSURLRequest (which still exists) by duplicating the existing NSURLRequest tests.

Whilst doing this I hit a crash when trying to mutate a value in a copied URLRequest. This is caused because, when we call _appyMutation for a non-uniquely referenced value, we create a new _MutableHandle. The _MutableHandle constructor calls copy() on the referent (in our case a NSMutableURLSession). Calling NSMutableURLSession.copy() actually returns a NSURLSession (as it inherits copy() from NSURLSession), which fails because its not a MutableType. I've fixed this by overriding copy in NSMutableURLSession to return mutableCopy().

[parkera]

@swift-ci please test linux

[phausler]

we should consider adding these structural tests into the overlay unit tests as well