Remove reference doesn't work natively with Reference.ref()

When trying to remove reference by using Reference.ref() then it fails:
```
References ref = ....
repo.git().references().remove(ref.ref())
```
Error message:
```
{"message":"Reference does not exist","documentation_url":"https://developer.github.com/v3/git/refs/#delete-a-reference"}
```
Root cause:
The uri is not correctly constructed as it contains two `refs` key words:
```
https://api.github.com/repos/ORG/REPO/git/refs/refs/heads/BRANCH/BRANCH
```