static mapping index not creating index for varchar > 255 (for MySQL)

### Steps to Reproduce

1. Create a domain class like this:

```
class MyDomain {

String attr

static constraints = {
attr(maxSize: 2048)
}
static mapping = {
attr index: 'my_idx'
}
}
```

2. run-app

3. Check the database, the index is not being created since 2048 exceeds the index max size (255).

### Expected Behaviour

The index creation code should check the maxSize and specify the size of the index like (this is for MySQL):

`INDEX `my_idx` (`attr`(255))`

### Actual Behaviour

It seems Grails is not checking the maxSize and tries to create the index without the size, and the database is not allowing it.

### Environment Information

- **Grails Version:** 2.5.6
- **JDK Version:** 1.7

Start from the static mapping index creation path and reproduce the MyDomain example with MySQL using run-app. Inspect the generated database index for attr, then verify that the completed behavior creates my_idx with the applicable prefix length for the maxSize value.

Written by the indexing model from the issue text.