Ensure `response.Request` is populated for responses from cache

https://github.com/github/hub/runs/397657829

features/api.feature

@@ -524,7 +524,7 @@ Feature: hub api
     Then the exit status should be 22
     And the stderr should contain exactly:
       """
-      
+
       You must authorize your token to access this organization:
       http://example.com?auth=HASH
       """

github/http.go

@@ -341,8 +341,9 @@ func (c *simpleClient) cacheRead(key string, req *http.Request) (res *http.Respo
 		}
 
 		res = &http.Response{
-			Body:   ioutil.NopCloser(bytes.NewBufferString(parts[1])),
-			Header: http.Header{},
+			Body:    ioutil.NopCloser(bytes.NewBufferString(parts[1])),
+			Header:  http.Header{},
+			Request: req,
 		}
 		headerLines := strings.Split(parts[0], "\r\n")
 		if len(headerLines) < 1 {