The HttpGremlinEndPointHandler only deals with MessageTextSerializer implementations and you can't configure multiple GraphSON ones because all of those implementations bind to application/json mimetype. I'm not even quite sure why a distinction exists anymore between "text" and "binary" serialization because we serialize to string only to turn around and convert to a ByteBuf which is what the binary version of the serializers already do.
This is a breaking change because the header mime type appended to requests generated from GraphSONMessageSerializerV2d0 changed from application/json to application/vnd.gremlin-v2.0+json.
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