RawResponse is never cached.

[joshua-rutherford]

The implementation of caching in #300 is not actually being used. There are the following issues:

1. RawResponse.marshaledResponse is never set so nil should always be returned within this block: https://github.com/envoyproxy/go-control-plane/blob/master/pkg/cache/v2/cache.go#L108. So things should be breaking but aren't because:
2. RawResponse.isResourceMarshaled will never be true with this check because the RawResponse.GetDiscoveryResponse method operates on the value of the raw response, not a pointer. Therefore, the RawResponse value in the method is not actually the same that the method is invoked upon (it is a copy).

The program below demonstrates issue number two above:

package main

import (
	"fmt"
)

type Structure struct {
	Boolean bool
}

func (s Structure) ToggleV() bool {
	s.Boolean = !s.Boolean
	return s.Boolean
}

func (s *Structure) ToggleP() bool {
	s.Boolean = !s.Boolean
	return s.Boolean
}

func main() {

	value := Structure{}

	for i := 0; i < 10; i++ {
		fmt.Printf("value: %t\n", value.ToggleV())
	}

	pointer := &Structure{}

	for i := 0; i < 10; i++ {
		fmt.Printf("pointer: %t\n", pointer.ToggleP())
	}
}

Which outputs:

value: true
value: true
value: true
value: true
value: true
value: true
value: true
value: true
value: true
value: true
pointer: true
pointer: false
pointer: true
pointer: false
pointer: true
pointer: false
pointer: true
pointer: false
pointer: true
pointer: false

[kyessenov]

Fixed.