Fix #6231 - Accept chunked encoding on start

Fixed using the @sfitts solution

I have a test for this, not commited since I can't seem to make go do the chunked encoding on the request, as such it passes no matter what.

Confirmed this fixes the issue manually.


```go
func TestConstainersStartChunkedEncodingHostConifg(t *testing.T) {
	eng := NewTestEngine(t)
	defer mkDaemonFromEngine(eng, t).Nuke()

	r := httptest.NewRecorder()

	var testData engine.Env
	testData.Set("Image", "docker-test-image")
	testData.SetAuto("Volumes", map[string]struct{}{"/foo": struct{}{}})
	testData.Set("Cmd", "true")
	jsonData := bytes.NewBuffer(nil)
	if err := testData.Encode(jsonData); err != nil {
		t.Fatal(err)
	}

	req, err := http.NewRequest("POST", "/containers/create?name=chunk_test", jsonData)
	if err != nil {
		t.Fatal(err)
	}

	req.Header.Add("Content-Type", "application/json")
	if err := server.ServeRequest(eng, api.APIVERSION, r, req); err != nil {
		t.Fatal(err)
	}
	assertHttpNotError(r, t)

	var testData2 engine.Env
	testData2.SetAuto("Binds", []string{"/tmp:/foo"})
	jsonData = bytes.NewBuffer(nil)
	if err := testData2.Encode(jsonData); err != nil {
		t.Fatal(err)
	}

	req, err = http.NewRequest("POST", "/containers/chunk_test/start", jsonData)
	if err != nil {
		t.Fatal(err)
	}

	req.Header.Add("Content-Type", "application/json")
	req.Header.Add("Transfer-Encoding", "chunked")
	if err := server.ServeRequest(eng, api.APIVERSION, r, req); err != nil {
		t.Fatal(err)
	}
	assertHttpNotError(r, t)

	type config struct {
		HostConfig struct {
			Binds []string
		}
	}

	req, err = http.NewRequest("GET", "/containers/chunk_test/json", nil)
	if err != nil {
		t.Fatal(err)
	}

	r2 := httptest.NewRecorder()
	req.Header.Add("Content-Type", "application/json")
	if err := server.ServeRequest(eng, api.APIVERSION, r2, req); err != nil {
		t.Fatal(err)
	}
	assertHttpNotError(r, t)

	c := config{}

	json.Unmarshal(r2.Body.Bytes(), &c)

	if c.HostConfig.Binds[0] != "/tmp:/foo" {
		t.Fatal("Chunked encoding not properly handled, execpted binds to be /tmp:/foo, got:", c.HostConfig.Binds[0])
	}
}
```
I think this check should happen before trying to read/decode anything.  `if contentlength == 0 { return err`