Don't run before/after download content hooks if password required (and not provided)

If you have a post / page / cpt with a password set on it, then you probably don't want someone to purchase it until the correct password has been provided.

`post_password_required()` checks to see if there is a password set on the post, and then if it's required.

* It returns `false` if no password is set
* It returns `false` if the password has already been provided
* It returns `true` if the password is set and has not been provided or was incorrect
Merge pull request #1685 from sc0ttkclark/patch-1

Don't run before/after download content hooks if password required (and not provided)