URI example does not abide by the `:path` option

``` ruby
class Test < Attributor::Struct
  attributes do
    attribute :href, Attributor::URI, path: %r{/foo/\d+}
  end
end

puts Test.example.dump
 => {:href=>"http://example.com/hydrocoralline/804302141"}
```

Although, it would correctly validate against it:

``` ruby
t = Test.load( {href: "/bar/123"} )
puts t.validate
=> $.href value (/bar/123) does not match path (/\/foo\/\d+/)
```
