MeshLineMaterial.copy fails

[jiehui-gelement]

Reproduce steps:

1. initialize a MeshLineMaterial instance
2. call .clone() on the MeshLineMaterial instance

the offending lines are below:

THREE.MeshLine/src/THREE.MeshLine.js

Lines 687 to 689 in b49ae60

	this.dashArray.copy(source.dashArray)
	this.dashOffset.copy(source.dashOffset)
	this.dashRatio.copy(source.dashRatio)