Change order of getWidthFrom calculation to prevent innaccurate widths

[dpedu]

This is to fix inaccurate widths being applied from getWidthFrom, under specific conditions:

• box-sizing of the container is border-box
• the container has left or right padding
• the element being stickied is being used as getWidthFrom

Under these conditions, when the sticky element is changed to position:fixed, it's width becomes equal to the width of the container + left/right padding on the container (due to border-box). This can lead to unexpected widths being applied to the stick element. Calculating the width from getWidthFrom before setting position:fixed fixes this issue.

[duzun]

I think commit a429b78f from PR #148 addresses this issue. In that commit the width is applied exactly once, at once with position: fixed.