Feature: New unmocked policies

[bstien]

Why?

Currently the user is not able to provide their own response for unmocked incoming requests – the only option is a hardcoded 404 and passthrough to real network.

The changes in this PR adds these two cases to UnmockPolicy:

• .mock(MockResponse)
• .fatalError

UnmockedPolicy.mock(MockResponse)

This one lets the user provide their own MockResponse for all unmocked requests. The single response will be used for all incoming requests where a mock can't be found – either because none is registered, or the queue is empty.

UnmockedPolicy.fatalError

This is an addition for stricter testing which will call fatalError() if there aren't any mocks for an incoming request. Usecases will be to i.e. create a hard assumption that all network requests are mocked or handled.

Closes #7

What?

• Add cases .mock(MockResponse) and .fatalError to UnmockedPolicy.
• Pull out shared code for sending response in HTTPURLMockProtocol.
• Add tests.
• Update README.

Show me

let httpMock = HTTPMock()

// Set your own response to unmocked requests.
httpMock.unmockedPolicy = .mock(.plaintext("Ouchie!"))

// Force a `fatalError()` call on incoming unmocked requests.
httpMock.unmockedPolicy = .fatalError