Fix typo in test name.
WIP: Added element_root option to ArraySerializer.

This includes the root of each element in the serialized array, so rather than:

[ { :id => 1 }, { :id => 2 } ]

You get:

[ :comment => { :id => 1 }, :comment => { :id => 2 } ]

Now, I'd like to add an option to `has_many` declaration, but I can't work out
where to add it in. Hence there is one failing test.