Backspacing to merge two same-styled lines changes the style of the second line

[Matt Hauck]

Steps to reproduce:

1. Create a page with two lines, both styled to h2. The second one with text, the first one either blank or with text.
2. Place the cursor at the start of the second line and backspace up to the first line

This changes the style of the second line and puts it in a span with a fixed font size, rather than just merging its style into the same style as the top line. I can see why you may want to do this when you have text on both lines and the styles of the two lines are different. However, in the case where the styles are the same, there should be no change. And when a line with text is merged with a blank line, it is the style of the line with text that should take precedence I think.

I have found a workaround that clearing the style of the first line before backspacing up to merge the two lines works, but this is a rather strange user experience when one just wants to get rid of an empty top line.

[Jakub Ś]

DUP of #9998.