Url validator doesn't accept localhost with port

### Task List
- [x] Steps to reproduce provided
- [x] Stacktrace (if present) provided
- [x] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)
### Steps to Reproduce
1. Create `url` validator for domain class string attribute.
2. Use localhost url with port, i.e. http://localhost:80.
3. Validate the domain class instance.
### Expected Behavior

I expect to have the localhost url with port valid. As a workaround, I've implemented custom validator with apache url validator.

```
import org.apache.commons.validator.routines.UrlValidator

validator: { val, obj ->
if (val) {
def urlValidator = new UrlValidator(UrlValidator.ALLOW_LOCAL_URLS)
if (!urlValidator.isValid(val)) {
return "typeMismatch.java.net.URL"
}
}
}
```
### Actual Behavior

It fails in validation, error message `url.invalid`.
### Environment Information
- **Operating System**: OS X 10.11.6
- **Grails Version:** 3.2.0
- **JDK Version:** 1.8.0_77
- **Container Version (If Applicable):** n/a
### Example Application

https://github.com/kuceram/url-validation

Start with the Grails URL validator behavior described in the issue and reproduce it using the linked kuceram/url-validation example application. Trace how `http://localhost:80` is handled, then verify that localhost URLs with ports are accepted without breaking existing URL validation cases.

Written by the indexing model from the issue text.