Change HttpFullFiledPromise constructor parameter to mixed

[aferrandini]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	fixes #123
License	MIT

What's in this PR?

Changes the HttpFulfilledPromise constructor parameter to mixed instead of ResponseInterface

Example Usage

return $response->then(
    function (ResponseInterface $response) use ($request) {
        if (200 <= $response->getStatusCode() && $response->getStatusCode() < 300) {
            return 'success';
        } 

        return 'fail';
    }
);

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix