Rework trailing slash urls

When using:

`trailing_slash = True` -- the default

I had mixed URLs for:

* POST (trailing slash)
* GET (trailing slash)
* GET id (no trailing slash)
* PUT id (no trailing slash)
* DELETE id (no trailing slash)

I don't know if this behaviour was intended.
Anyway I have prepared this PR to adapt the behaviour, I will close it if it does not make sense

Other **note**. With this patch if someone uses `trailing_slash = True` it will get consistent trailing slashes and `301` redirects to trailing slash urls

Other than the change itself, I adapted the pre-existing tests that were failing after introducing 804f20b

Let me know thanks ;)