[BUG] Using multiple instances of the middleware with different rules is not possible

[Nicolai-]

Detailed description

With the new feature implemented in #33 which we appreciate being implemented! :)
I expected to solve our use case by adding 2 instances of the Middleware, but that does not work.

Ex.
Instance 1 ( We wan't to retry on all 429 no matter the method):

GuzzleRetryMiddleware::factory([
    'retry_on_status' => ['429']
])

Instance 2 (Retry on all GET requests for another set of response codes):

GuzzleRetryMiddleware::factory([
    'retry_on_methods' => ['GET'],
    'retry_on_status' => ['500', '503', '504']
])

The issue(from what I can tell) is that the "combined" options created as the first thing inside __invoke from the middleware is provided to $next (the second middleware), which then overwrites the default options (The second set of rules for the other middleware).