Embed_js returns invalid URL

Calling `get_embedjs_url` returns `//gp-gw/js/embed.js` which is a non-sense. 

Looking to the code of the method:

```
@property
def get_embedjs_url(self) -> str:
    return self.gopay.base_url[-4] + "/gp-gw/js/embed.js"
```

It probably should be changed to:

```
@property
def get_embedjs_url(self) -> str:
    return self.gopay.base_url[:-4] + "/gp-gw/js/embed.js"
```




