In UniVRM v0.45 invalid glTF json values like
`meshes[].primitives[].targets[].TANGENT = -1` had been removed
by UniGLTF.JsonSerializableBase.SerializeMembers(). But it was disabled
unexpectedly since v0.46's following commit:
a85b9699e7 (diff-6d81c3ba04eb19e159e64767f9dd1325L36)
Now call UniGLTF.GLTFJsonFormatter.GLTFValue() explicitly to run serializable's
SerializeMembers() for removing those values that restores UniVRM v0.45
behavior without reflections.
Fixes#105