httpclient Response object should reference original Request

[daehee]

For feature parity with examples of HTTP implementations below, httpclient module's Response object should have a reference to the original prepared Request.

References

• From Go standard library: "Request is the request that was sent to obtain this Response."(https://golang.org/pkg/net/http/#Response)
• From Python requests library: "The PreparedRequest object to which this is a response."(https://requests.readthedocs.io/en/latest/api/#requests.Response.request)

[dom96]

Sure, I suppose it can't hurt to have this.

[daehee]

PR'd #16691