Add hasMiddlewareGroup and getMiddlewareGroups methods

[MikeMnD]

Handy method to be used when you want to register Middleware from a ServiceProvider and use:
prependMiddlewareToGroup
pushMiddlewareToGroup
Because they gracefully fail to add a middleware if a group is not existing first.
Also you can use:
middlewareGroup($name, array $middleware)
but still you can't be sure if you are overwriting an existing group ... since there is no way to check if the group already exist.
Also adding getMiddlewareGroups method just for convenience if you want to check multiple groups or do something in a foreach loop.