Original mistake: assume that the default behaviour of the serializer was to transform the keys name in camelCase. Default is dashed or spinal-case as expressed here in the keyForAttribute method section of the docs (so straightforward…).
By the way, attrs seems to not work so I tried keyForAttribute.
I need to thank @novascape for this that, actually, override the default transformation and returns the keys name from your attributes object in your JSON payload as they are.
In every case, bookmark the methods to manipulate strings in ember, here.