Does not allow serialization for purposes of Creating Resources

[xn]

Expected behavior vs actual behavior

I expect to be able to serialize an object without an id for the purpose creating a resource.
Current behavior returns: {data: null}

Steps to reproduce

let(:user_attributes) { FactoryGirl.build(:user) }
post "/v1/users", params: ActiveModelSerializers::Adapter::JsonApi.new(UserSerializer.new(user_attributes)).serializable_hash

Environment

ActiveModelSerializers Version (commit ref if not on tag):
Everything south of 73eae19b3d0b8a3c03fda566a208eeee6720beed

Additonal helpful information

(e.g., Gemfile.lock, configurations, PR containing a failing test, git bisect results)

From the spec:
Creating Resources
A resource can be created by sending a POST request to a URL that represents a collection of resources. The request MUST include a single resource object as primary data. The resource object MUST contain at least a type member.
This is implying that the resource object MAY contain an id, not MUST.

Also here:
#2132

[bf4]

Fixed by #2216