Fix removeRelativePath

Instead of checking the existence of relative_path (without /) anywhere on the url, it really should check that the url only starts with the relative_path.

Example

```
relative_path = /forum
without / = forum
url = /category/forum/5
removeRelativePath = ory/forum/5
```
Merge pull request #1658 from char101/patch-4

Fix removeRelativePath