Api BaseUrl is not honored by `do` method

I'm using `gopencils` like this:
```
res := gopencils.Api("http://host.name/path/to/api", nil)
res.Res("resname").Id("id123", &data).Get()
```

Before commit ce8e830 it was working perfectly and that code accesses
`http://host.name/path/to/api/resname/id123` URL.

After specified commit code will go to the `http://hostname/resname/id123`, which is incorrect, because of `Path` component of `r.Api.BaseUrl` gets overwritted by the resource URL.

Proposed path fixes that behavior, so resource URL will be added to the Api Path.