Changing column width does not work with Sproutcore 1.11

[dstevick41]

When attempting to change the width of a column the header view grows and shrinks erratically. After investigating it appears the event.pageX value used the first time SC.TableHeaderView:mouseDragged() is called is undefined causing the newWidth calculation to generate erratic values.

This can be solved by saving the X position in the mouseDown() function then checking the event.pageX for undefined in mouseDragged(). If so use the saved X position.