box zoom should not reset bearing

[jfirebaugh]

Currently the box zoom behavior is to reset the bearing to 0. This means that box zooming doesn't actually zoom to the box if the map was rotated. Instead, it should keep the current bearing.

[1ec5]

#1340 tracks a simple fix for this issue. #1338 tracks a fix that would correctly fit to a rotated bounds.

[andrewharvey]

I presume this would mean that map.fitBounds(map.getBounds()) on a rotated map would retain the map's rotation rather than resetting it to 0.

[lucaswoj]

Merged with #2112