Remove normalized association from association cache for marshaling.

has_many associations use a CollectionProxy to act as a lazy loaded
array so that it can be used as an array or can be used to build a query.
This collection proxy will delegate all methods to either the association
or to a new scope, including respond_to?.

A normalized has_may association will have the association stored in the
association cache, since we cache the associated ids, but the association
will not be loaded.

When marshal dumping a record with normalized has_many associations, the
marshal code will call respond_to? on the collection proxy, which will load
the association to delegate respond_to? to it.  This can also corrupt
the marshal dump since it will set a new instance variable while dumping
the association, so will get interpreted as being outside the
objects instance variables.
Assert that normalized associations are loaded on fetch.

Previously this was happening through Marshal.load calling respond_to? on
the association.
Merge pull request #55 from Shopify/remove-normalized-association-cache

Remove normalized association from association cache for marshaling.