Check URI scheme for src attributes

Me again... This time with a bugfix.

> As of version 1.0.3, URI scheme checking is performed on all href and src tag attributes.

 "src" attributes are not being checked:

```perl
my $hr = HTML::Restrict->new(
    rules => {
        a      => [qw( href )],
        img    => [qw( src  )],
    },
);

$hr->set_uri_schemes( [ undef, 'http', 'https' ] );

say $hr->process('<a href="file:/some/file">click here</a>');
# <a>click here</a>

say $hr->process('<img src="file:/some/file">');
# <img src="file:/some/file">
```