Interceptors cannot short-circuit request/response

There is an asymmetry between interceptors in the _http-client_ module and the _http-fetch_ module.

The **http-fetch** module has the option to short-circuit the request/response process using an interceptor. You can return a `Response` object (or a `Promise`) from a request interceptor method and the fetch module will simply treat that response as the result of the process and switch directly to the _response_ half of the pipeline (chain) without actually making any network calls.

This functionality is missing from the equivalent http-client module. This is clearly an unintentional asymmetry - and a feature that I will find very useful when writing a request batching plugin. It also looks to be only a couple of lines to change, so I intend to offer a pull request.

Compare the interceptor handling in the http-fetch and http-client modules, starting with the request interceptor path in each. Confirm how a returned Response or Promise bypasses the network call in http-fetch, then verify that http-client reaches the response half of its pipeline with the same behavior.

Written by the indexing model from the issue text.