Decode HTML characters for email subject lines

[betzster]

Since email subject lines are plaintext, we should decode database values that we use there. All the examples I found are related to blogname.

• ​http://core.trac.wordpress.org/browser/trunk/src/wp-admin/includes/ms.php?rev=25472#L214
• ​http://core.trac.wordpress.org/browser/trunk/src/wp-admin/includes/ms.php?rev=25472#L267
• ​http://core.trac.wordpress.org/browser/trunk/src/wp-admin/ms-delete-site.php?rev=25472#L72
• ​http://core.trac.wordpress.org/browser/trunk/src/wp-admin/user-new.php?rev=25472#L88

Related: #9913

[jeremyfelt]

It looks like we use wp_specialchars_decode() in a few places with email. Would it make sense to go through all wp_mail() uses and make sure that things are formatted properly? Would decoding cause any other issues?

[nacin]

Replying to jeremyfelt:

It looks like we use wp_specialchars_decode() in a few places with email. Would it make sense to go through all wp_mail() uses and make sure that things are formatted properly?

I'd say so.

[tlovett1]

wp_specialchars_decode() database values in wp_mail subject lines

[jeremyfelt]

25346.diff​ covers a few additional cases to those addressed in @tlovett1's initial patch. This includes a few instances where $blogname may have already been run through wp_specialchars_decode(), but then was passed to a filter before being included as the subject.

[nacin]

In 27801:

Always decode special characters for email subjects.

props tlovett1, jeremyfelt.
fixes #25346.