Custom URLProtocol doesn't work with firebase calls

[jerbeers]

• Xcode version: 8.3.3 and 9.0 beta 3
• Firebase SDK version: 4.0.0
• Firebase Product: auth, database

I use a custom URLProtocol class in my tests. If I register my URLProtocol class and use URLSession to load a URL, the custom class is used, but if I make a call to any firebase API, the custom class doesn't get used.

Steps to reproduce:

1. Write a URLProtocol subclass, implement canInit(with request:), set a breakpoint here
2. In the setUp() method of your test class, add URLProtocol.registerClass(CustomURLProtocol.self)
3. Write a test and load any URL using URLSession.shared.dataTask(with:)
4. Run the test and notice the breakpoint in canInit gets hit
5. Change the test to call Auth.auth().createUser or Database.database().reference().child(path).setValue
6. Run the test and notice the breakpoint in canInit does not get hit

[jerbeers]

Sorry, I think I misunderstood how registerClass would work. It works for the shared instance of URLSession, but not one with a set configuration. I'll look for another way to test.