Using stripslashes malforms arrays for post metadata

[jacobsantos]

In the database, the gallery has _wp_attachment_metadata set as "Array" this means that somewhere in the execution path, the meta data value was converted to a string. Following the execution has found that stripslashes() is the cause of the malformed array meta data values.

The patch solves this problem and keeps the stripslashes for strings. The recommendation is to remove the requirement that strings have slashes added in order to simplify the code.

[jacobsantos]

Fix for malformed array in database based off of r9124

[ShaneF]

This is a major error. It even fixes problems I was having in #7607.

[azaozz]

Why not just use stripslashes_deep(). It works for both arrays and strings.

[azaozz]

(In [9129]) Fix stripslashes for post metadata, phpDoc updates for media.php, props jacobsantos, fixes #7871

[ShaneF]

I am going to test this later to make sure.