官方解释：

This method closes the current subpath by creating a line segment between 
the first and last points in the subpath. 
This method subsequently updates the current point to the end of 
the newly created line segment, 
which is also the first point in the now closed subpath.

调用该方法会添加一条线，线的终点即整个路径最开始的起点。

1、没有调用closePath
UIBezierPath closePath_closePath
2、调用closePath
UIBezierPath closePath_sed_02
该方法等价于：[bezierPath addLineToPoint:startPoint];