Provide compatibility with CacheInterface

[shgysk8zer0]

Check that Requests & Responses are the same before and after serialize() & unserialize().

In other words:

<?php
$req = new Request();
$resp = new Response();
/**
 * Build up $req & $resp
 */

unserialize(serialize($req)); // Same as original Request?
unserialize(serialize($resp)); // Same as original Response?

One issue that I suspect difficulty in encountering is whether or not CURLFile can be serialized. I could easily create a custom class that just serializes the filename, type, & postname, but cannot guarantee that the file will still exist when unserialized.

Tasks:

• Ensure compatibility with serialize() & unserialize() for caching
• Determine cache keys from request URL & probably Accept & Vary headers
• Implement a serializable file class extending CURLFile
• Determine method of setting expiration for cache (response headers, maybe?)
• Handle caching by an init preference, similar to JS's Request.cache preference
• Update example in README.md

[shgysk8zer0]

Perhaps I could provide cache key using Request::__toString(), allowing:

if ($cache->has($req)) {
    $resp = $cache->get($req);
}

Request could then provide methods taking expiration & fallback args and work somewhat more directly with CacheInterface.

[shgysk8zer0]

Due to the complexity of HTTP caching, I do not think that the basic CacheInterface can be compatible. I need to be able to check ETags, make conditional requests, utilize Cache-Control headers, etc.