Added test for horizontal lines

Notice that the expansion of the line width depends on the order of the
points:
  * If the bigger axis value is provided as the *second* point the line
    expand first to the *positive* side of the axis.
  * If the bigger axis value is provided as the *first* point the line
    expand first to the *negative* side of the axis.
  * If the line's width is odd this doesn't matter, as the line will
    expand the same amount to both sides.

This behavior should be consistent in both horizontal and vertical lines.
Added test for vertical lines.

The behavior is consistent with horizontal lines, see previous commit
for details.