PSR-7 responses shouldn't be considered as serializable #81
Description
In src/CachePlugin.php, PSR-7 responses are serialized as is, except the response body, which is serialized separately.
Recently the PHP engine started to throw deprecations when a resource is found in a serialized data structure.
And this happens to PSR-7 responses: whatever the implementation, their body is a represented as a stream resource internally.
Also, from an abstraction PoV, nothing says in PSR-7 that responses are serializable, which means interop should consider them as non-serializable.
The plugin should be improved to not store PSR-7 objects directly, but intermediate representations that fit a generic cache backend.