Mock domain collections fail to save properly when combined with @EqualsAndHashcode

Example: https://github.com/joemccall86/grails-domain-equals-hashcode-test

If there's a one-to-many relationship between 2 simple GORM classes, and the many-side has the `@EqualsAndHashcode` annotation, unit tests do not behave as expected. Specifically calling `addTo` on the "one" side will append the collection, and subsequently calling `save` will append the collection again. See the example attached. Important code is copied below for your convenience.

Domain classes:

``` groovy
class Author {

static hasMany = [
books: Book
]

static constraints = {
}
}
```

``` groovy
import groovy.transform.EqualsAndHashCode

@EqualsAndHashCode
class Book {

static constraints = {
}

static belongsTo = [
author: Author
]
}
```

Test:

``` groovy
@Mock([
Book,
Author
])
@TestFor(Author)
class AuthorSpec extends Specification {

void "test that we can add to books"() {
given: 'an author'
def author = new Author().save(flush: true, failOnError: true)

and: 'a book is added'
author.addToBooks(new Book())

when: 'the author is saved'
author.save(failOnError: true)

then: 'the author only has 1 book'
author.books.size() == 1
}
}
```

The test fails with:

```
Condition not satisfied:

author.books.size() == 1
| | | |
| | 2 false
| [org.grails.Book : 1, org.grails.Book : 1]
org.grails.Author : 1

at org.grails.AuthorSpec.test that we can add to books(AuthorSpec.groovy:34)
```

I can only speculate as to why it's happening. This test does not seem to break when run as an integration test, indicating it's a problem with `@Mock`-ed domain classses.

Start with the attached example repository and its AuthorSpec test, reproducing the one-to-many Author/Book case with @Mock and @EqualsAndHashCode. Compare the unit-test behavior with the integration-test behavior described in the issue. Done means saving after addToBooks leaves exactly one book and the regression test passes.

Written by the indexing model from the issue text.