SAL: use Post ID instead of Post Object in current_user_can

[jeherve]

Changes proposed in this Pull Request:

current_user_can only accepts object IDs as the optional second param; passing anything else (like an instance of Jetpack_Post like we did) will cause issues.

This was reported in 2184839-zen by a site owner using Polylang Pro.

Testing instructions:

I could not reproduce the Fatal myself, but something like this may help:

• Connect your test site to WordPress.com.
• Install the Polylang plugin.
• Add the site to your mobile WordPress app.
• Try to access the Posts screen in the app.
• Check your site's error logs and make sure you do not see any errors like those:

PHP Catchable fatal error: Object of class Jetpack_Post could not be converted to string in /wp-content/plugins/polylang-pro/include/filters.php on line 305 

Proposed changelog entry for your changes:

• WordPress.com API: avoid Fatal Errors when fetching posts from the mobile apps.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello jeherve! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D31068-code before merging this PR. Thank you!

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: August 6, 2019.
Scheduled code freeze: July 30, 2019

Generated by 🚫 dangerJS against aad988e

[Chouby]

@jeherve Thank you for the PR!
Some precision if you need to reproduce: You'll need to setup a privacy policy page to pass the first part of the test in https://github.com/polylang/polylang/blob/2.6.2/include/filters.php#L305.
FYI, the goal of the function is to disallow non-admins ( 'manage_privacy_options' capability ) to edit / delete the translations of the privacy policy page as WP does for the original page.

[tyxla]

LGTM 🚢

[zinigor]

I have been able to reproduce this problem using the plugin code from GitHub and a request to the v1.1 post update endpoint to the privacy page via WordPress.com REST API. LGTM!

[jeherve]

Cherry-picked to branch-7.6 in 6e2e2a1