Handle header parameters and non-GET requests

[tidoust]

The code assumes that only GET requests will be issued, but the package could perhaps be used in situations that require sending POST and other types of HTTP requests.

In the same vein, while the code does send additional HTTP headers provided in the call to fetch, it does not take these headers into account to search the cache entry. Sending the same GET request with two different Authorization HTTP header will likely return two different responses!

At a minimum, the README should be clear that the package does not support these scenarios for now.