add_query_arg() should handle encoded input URLs

[kkmuffme]

When passing an already encoded URL to add_query_arg, it will make existing query args after # to be an anchor.
Since add_query_arg expects URL encoded params for key/value, people often assume this is the case for the URL too - that results in an unexpected result though.
Especially since it says:

Values are expected to be encoded appropriately with urlencode() or rawurlencode().

and

$value Optional. Either a query variable value, or a URL to act upon.

<?php
echo add_query_arg('foo', 'bar', '/wp-admin/admin.php?page=hello&#038;tab=world&#038;abc=xyz' );

will result in

'/wp-admin/admin.php?page=hello&foo=bar#038;tab=world&abc=xyz'

I guess add_query_arg should either add a "doing it wrong" when passing an encoded URL or automatically, correctly handle encoded URLs.

Trac ticket: #63462