Tighter type checking on passed in nodegit methods 

As seen here: https://github.com/nodegit/nodegit/issues/1120

Generated code is as such

```
NAN_METHOD(GitTag::List)
{

  if (info.Length() == 0 || !info[0]->IsObject()) {
    return Nan::ThrowError("Repository repo is required.");
  }
```

Where info[0] is meant to be a GitRepo object. Templates need to be fixed up to support this. 
