make catmaid_login more robust to variations in the server URL.

```r
> v14=catmaid_login(server='https://neuropil.janelia.org/tracing/fafb/v14/')
> v14=catmaid_login(server='https://neuropil.janelia.org/tracing/fafb/v14')
Error in catmaid_login(server = "https://neuropil.janelia.org/tracing/fafb/v14") : 
  Unauthorized (HTTP 401).
```