Future-proof the Rc::as_ptr too

[vorner]

Forgotten part of fd04e208d52331563b5f5d38a7b021f936c36587.

[codecov-commenter]

Codecov Report

Merging #87 (7af4c73) into master (634dd80) will decrease coverage by 0.19%. The diff coverage is 0.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #87 +/- ## ========================================== - Coverage 80.44% 80.24% -0.20% ========================================== Files 16 16 Lines 803 805 +2 ========================================== Hits 646 646 - Misses 157 159 +2 ``` | [Impacted Files](https://codecov.io/gh/vorner/arc-swap/pull/87?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michal+%27vorner%27+Vaner) | Coverage Δ | | |---|---|---| | [src/ref\_cnt.rs](https://codecov.io/gh/vorner/arc-swap/pull/87/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michal+%27vorner%27+Vaner#diff-c3JjL3JlZl9jbnQucnM=) | `66.66% <0.00%> (-4.77%)` | :arrow_down: |