InstancedInterleavedBuffer: Fix incorrect clone result

[gkjohnson]

Related issue: --

Description

When calling InstancedInterleavedBuffer.clone() an InterleavedBuffer was returned:

instanceBuffer = new THREE.InstancedInterleavedBuffer( new Float32Array( [ 1, 2, 3 ] ), 3, 1 );
a = new THREE.InstancedBufferGeometry()
a.setAttribute( 'test', new THREE.InterleavedBufferAttribute( instanceBuffer, 3, 0 ) );

b = a.clone()

console.log( a.attributes.test.data.isInstancedInterleavedBuffer, b.attributes.test.data.isInstancedInterleavedBuffer );

// BEFORE: true, undefined
// AFTER: true, true

[mrdoob]

Thanks!