bug: weekdayHeader shows even weekdayHeaderEnabled is diable

[scorpiozj]

There is a slender line of weekdayHeader (I enable the text into green) of the demo.
It happens in iPhone6 Device running iOS8.

Analyze:

1. I set the background of weekdayHeader into red and the text label into green. However, it only shows the green text.
2. I log the frame of weekdayHeader and its subview, all the heights are 0. I doubt it may be a bug of AutoLayout.

Currently I use setHidden: to fix it and hope there is a better way.