Serialization of fetch request needs implementation

[ksaaskil]

unmock-fetch package contains a method for creating a "fake" fetch that's expected to implement the Fetch API. When a user creates a call with the fake fetch, the request should be serialized to an ISerializedRequest object. There's a crude implementation for serialize.ts that does not yet handle request headers or body.

I have written tests for the serialization features. They're skipped as they don't work yet, so the task would be to make those tests pass.

Note that the fetch interface also does not implement the case where "url" is not a plain string but a Request object. That would be a task for another PR.

In summary, we would need:

• Serializing request headers (unmock-fetch: Implement serialization of request headers. #347)
• Filling "path" with the full path containing the search parameters
• Serializing request body
• Implementing serialization for the case where the input "url" is a Request object

Each of these could be a PR of their own.