Prepare for content-type negotiation

Combine the fields that will be used for content transformation
(content-type, codec, and group version) into a single struct in client,
and then pass that struct into the rest client and request. Set the
content-type when sending requests to the server, and accept the content
type as primary.

Will form the foundation for content-negotiation via the client.

pkg/client/unversioned/testclient/simple/simple_testclient.go

@@ -76,15 +76,15 @@ func (c *Client) Setup(t *testing.T) *Client {
 	c.server = httptest.NewServer(c.handler)
 	if c.Client == nil {
 		c.Client = client.NewOrDie(&client.Config{
-			Host:         c.server.URL,
-			GroupVersion: testapi.Default.GroupVersion(),
+			Host:          c.server.URL,
+			ContentConfig: client.ContentConfig{GroupVersion: testapi.Default.GroupVersion()},
 		})
 
 		// TODO: caesarxuchao: hacky way to specify version of Experimental client.
 		// We will fix this by supporting multiple group versions in Config
 		c.ExtensionsClient = client.NewExtensionsOrDie(&client.Config{
-			Host:         c.server.URL,
-			GroupVersion: testapi.Extensions.GroupVersion(),
+			Host:          c.server.URL,
+			ContentConfig: client.ContentConfig{GroupVersion: testapi.Extensions.GroupVersion()},
 		})
 	}
 	c.QueryValidator = map[string]func(string, string) bool{}