Fix Failed Saving Pull Job Script File

[4roring]

close #62 fixes #62

[jordengit]

Fix script from xml in pullJobScript function is better.

private async pullJobScript(item: PipelineTreeItem) {
....
let script = root.definition[0].script;
if (undefined === script) {
....
}
script = script.toString();
....
}

[ariyuan]

close #62 fixes #62

This change works for me