User's ID should alway be integer

When user that doesn't have `list_users` capability request its own data using `/users/me`, they are redirected to `/users/<ID>`, which means ID is passed as a string to `WP_JSON_Users::get_user()`. As `get_current_user_id()` returns integer, and there is strict comparison, this will fail.

User's ID should be converted to integer, or there shouldn't be strict comparison.
Merge pull request #333 from dimadin/patch-1

User's ID should alway be integer.  Closes #333.