Fix ArraySerializer's output when falling back on DefaultSerializer

Change ArraySerializer's initialization of DefaultSerializer to pass
:root => false in order to avoid adding root keys to each of the
items in the array. This ensures consistent results between serializing
arrays of objects with serializers and arrays of objects without
serializers. Fixes #495.
Merge pull request #538 from mayhewluke/fix-array-output

Fix ArraySerializer's output when falling back on DefaultSerializer