Column titles do not persist as section labels when no section label is specified

[jeremyfelt]

I dug waaaaay back to Spine 0.20.0 before I gave up and I'm not sure this ever worked, but here it goes.

Section Labels (most useful when the section is minimized) should be populated via:

1. The section label input provided in the section configuration.
2. If not available, the header of the first column in that section.
3. If not available, the header of the second column in that section.
4. If not available, empty.

At the moment, 1 works fine and has a better interface as of #303. 2 appears to work while you start typing the column header, but the section label disappears when the page is saved. I don't think 3 is accounted for at all. And 4 kind of just works by default.

[jeremyfelt]

Fixed in fbed9cc