Merging an empty with a sided non-empty feature breaks the non-empty feature's side directionality

[RudyTheDev]

Scenario: draw a guard rail (case 1 in screenshot), then draw another empty line and connect them going in opposite direction (case 2). Shift-select the guard rail (case 3) and combine the features expecting the new line to become the guard rail (case 4). Observe that the "new" guard rail actually changes the side/direction (case 5).

This is fine when features know how to reverse themselves (and it looks like iD now does this great), but stuff like guard rails are side-dependent and not reversible.