next URL in paginated results should keep protocol (ie https if 1st request is https)

https://tark.ensembl.org/api/transcript/?loc_end=36530514&loc_region=7&loc_start=36530417

Returns:

```
"next": "http://tark.ensembl.org/api/transcript/?loc_end=36530514&loc_region=7&loc_start=36530417&page=2",
```

When generating the next link, it should keep the current protocol