Update usage example for joins_many attribute and add some tests

The tutorial states that you can use say:
```
foreach ( $novel->reviews ) { ...
# $_ is now a MyApp::Schema::Review object # NOT TRUE
```
and that this will give you the MyApp::Schema::Review objects. But this is incorrect, you need to call the "all" method for this to work.
```
foreach ( $novel->reviews->all ) { ...
# Now $_ really is a MyApp::Schema::Review
```

I've also added a number of tests to prove this.