Respect the http.RoundTripper contract

[CAFxX]

Fixes #20

Without this, it's impossible to use httpcache together with this package, as httpcache expects the parent RoundTripper to respect the RoundTripper contract:

	// RoundTrip should not modify the request, except for
	// consuming and closing the Request's Body. RoundTrip may
	// read fields of the request in a separate goroutine. 

ghinstallation currently does not comply with the contract, as it modifies the request headers (in addition to not closing the body in case of errors).

As a result of this, no request will ever result in a hit in the cache because ghinstallation modifies some of the request headers that the Github API includes in the Vary response header (Accept, Authorization).

As a hacky workaround until this bug is fixed in ghinstallation, you can add an intermediate roundtripper between httpcache and ghinstallation to clone the request, so that ghinstallation modifies the clone instead of the request passed by httpcache:

	// itr is the ghinstallation RoundTripper

	dtr := RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
		return itr.RoundTrip(req.Clone(req.Context()))
	})

	// use dtr as the Transport for httpcache

[CAFxX]

@bradleyfalzon any chance to get this looked at? 🙏🏻

[bradleyfalzon]

Apologise, this looks good to me and thanks for correcting these issues!