Removed feature string -> path feature

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #13
| License       | MIT
| Doc PR        | never document on purpose

I choose to remove the feature from both ContentObjectProvider and
ContentMethodProvider.
I choose to still split in the ContentDateTimeProvider, as it'll never
have the problem with sluggifing.

I also fixed the test, as it didn't test anything before...