JSON API: Plugins endpoint breaks characters in plugin action links

[tyxla]

It appears that the /sites/%s/plugins/%s endpoint does not work well with Cyrillic characters. This will probably happen with many other characters.

Steps to reproduce the issue

• Change the language of your site to Bulgarian.
• Connect your site to WordPress.com.
• Go to /plugins/jetpack/$site in WordPress.com.

What I expected

• To see the proper link text here (Настройки and Поддръжка).

What happened instead

• You will notice that the encoding of the "Settings" and "Support" is broken.

Reason

A quick investigation has shown that the plugins endpoint provides the broken texts:

This appears to be working just fine with the English action link texts.

It seems this has been introduced when the action links were introduced in #6267.