GLTFExporter: force node names to be strings.
If an object name is a number it is currently copied to the gltfNode as a number. This results in gltf validator errors:
"code": "TYPE_MISMATCH",
"message": "Type mismatch. Property value 26 is not a 'string'.",
"severity": 0,
"pointer": "/nodes/1/name"
This change ensures gltfNode.name is a string in all cases.