[HttpFoundation][NamespacedAttributeBag] Refactoring of resolveKey() method

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |

Just code enhancement. Low priority.

1) We don't need to call strlen($name) as last argument of substr() function
2) We don't need to call strrpos (or strpos) two times.