MergePullRequest update

[xabufr]

It seems that MergePullRequest is out of date for gitea 1.6.
We have this: https://github.com/go-gitea/go-sdk/blob/master/gitea/pull.go#L136
But gitea need additional parameters to merge: https://github.com/go-gitea/gitea/blob/v1.6.4/modules/auth/repo_form.go#L362
Minimalist valid requests looks like:

{
    "do": "merge"
}