Introduce WP_User::has_prop_val() magic method

[wpsmith]

When using WP_User::has_prop( $key ), I believe the expectation is that if the property has a value, it would return true. However, it is returning true if the key is set or if the key exists. So if one uses it like WP_User::has_prop( 'user_url' ), it returns true regardless, simply because that's a standard WordPress user field.

So, keeping that functionality for checking even custom properties, I thought it may be beneficial to add a magic method for checking whether a property is set ( WP_User::has_prop( $key ) ) and has a value ( WP_User::get( $key ) ).

Thus, this ticket desires to add two methods: has_prop_val() & __hasval()

Magic methods aren't something you can arbitrarily create. They're a predefined set of special constructs in the PHP language. __hasval() is not one of them: ​http://php.net/manual/en/language.oop5.magic.php