Fix bug with incorrect request payload for groups (Branch Restrictions) (#172)
Fix bug with incorrect request payload for groups (Branch Restrictions) (#172)

The API expects the group slug to be passed in, without this, API returns a HTTP 400 error. Also, fixed `branchRestrictionsBodyGroup` struct, specifically the inner `links` struct which accidentally included `full_slug` & `slug` - both of which should be at the root level.

Previously a request with this payload
```
{"kind":"push","pattern":"develop","links":{"self":{"href":""}},"value":null,"id":0,"users":null,"groups":[{"name":"za-test"}]}
```

Would return
```
{"type":"error","error":{"message":"malformed groups"}}
```

With the fixed payload, we now get a success response.
```
{"kind":"push","pattern":"develop","links":{"self":{"href":""}},"value":null,"id":0,"users":null,"groups":[{"slug":"za-test"}]}
```
ktrysmt/go-bitbucket