Fixing bug where WP_User->roles array does not start with a 0 index

[desrosj]

See #814 for context.

[tylerhcarter]

@desrosj Attempting to reproduce the issue. Do you know under what circumstances you were not getting a 0 index array?

[desrosj]

I believe I have it narrowed down to the user having a capability added to their account prior to their current role. The key in the roles property in WP_User appears to correspond with it's placement in the caps property. The site that I noticed this error on was assigning capabilities to users that were not in a role in order to allow access to content in a drip manner.

To reproduce:

1. Create new user.
2. With code, or a plugin that allows you to add custom capabilities, add a capability that is not a role to the user (in our scenario it was $user->add_cap( 'custom_cap' ).
3. Change the user's role (can be with code or in the admin).

You should then notice the error then. If you print out or log the WP_User object, you will notice the order of items in $user->caps and the keys in $user->roles.

[tylerhcarter]

Reproduced the bug and verified the code fix. Refreshing the commit in pr/815