Behave IAM policy group methods like user policy

At this moment, `Fog::AWS::IAM.get_group_policy` returns group policy in a different way that `Fog::AWS::IAM.get_group_policy` despite of API docs say the same.

In `Fog::AWS::IAM.get_group_policy` the data isn't contained in a `Policy` key and `PolicyDocument` returns a flat string with JSON data, while `Fog::AWS::IAM.get_user_policy` returns the JSON data inside a `Hash`.

To accomplish this the following changes are done:

- [x] Create mock group with `policies` and `created_at` keys
- [x] Create mock methods for `get_group_policy` and `put_group_policy` requests, enable them in tests
- [x] Run specs for `get_group_policy`
- [x] Return `PolicyDocument` inside `Policy` section and return `PolicyDocument` as a Hash in `get_group_policy` parser