Fix interceptors

[omgaXD]

This PR makes onRequest interceptors more flexible and predictable and fixes #6

• Added tests for interceptors:
• • onRequest:
• • • test interception
• • • test that modifying the arguments of the interceptor is a no-op
• • • test that returning new values for the input url and request init actually alters the request
• • onResponse:
• • • test interception
• Modified onRequest type to support returning void / promise of void (means preserving the original request as-is) and to support returning new values for input and init / promise of input and init (means modifying the request).
• Modified the docs to reflect this feature correctly (it was previously claiming that modifying onRequest args directly does something, which is not the case).

[omgaXD]

I accidentally started a PR into the original repo - meant to PR into my fork! Sorry about that.
Note that if you're interested in accepting the changes regardless, I did not update the changelog. I didn't change anything in the react-query thing as well.