Fix #148 igbinary_serialize cannot serialize resource(stream)

[xavierleune]

When using igbinary_serialize in the process to store a CacheEntry, we can get the following warning from PHP: Message: igbinary_serialize(): Cannot serialize resource(stream) and resources may be converted to objects that cannot be serialized in future php releases. Serializing the value as null instead. CacheEntry already mitigates the fact that a resource cannot be serialized, but the Response object still contains the stream so the warning is still there.

As a workaround, I propose to replace the resource with an instance of GuzzleHttp\Psr7\PumpStream, the only implementation of Psr\Http\Message\StreamInterface not using a resource in its internal, as it's meant to fetch data from a callback. In this implementation I've chosen to create a BodyStore, with the very minimal logic to return expected data when invoked.

By using this new implementation of Psr\Http\Message\StreamInterface for the body, we can totally remove the need to manipulate the Response on __wakeup

[Kevinrob]

Thanks for this PR.
LGTM