fix(fetch): type narrowing with default response types

[mnahkies]

relates #439, and includes changes from #444 as the actual fix

• adds failing test showing that type narrowing isn't working as expected for fetch clients when a default response is present
• adjusts the types to exclude specific status codes from the default response branch. The specification states that the default response is only to be used for responses not otherwise declared:

The documentation of responses other than the ones declared for specific HTTP response codes. Use this field to cover undeclared responses.