By changing a file to trigger the action

- Add extra condition in `topspace--post-command` before executing
  `beginning-of-visual-line`, which can be time consuming
- Update comment in `topspace--text` to reflect the fact that
  it is likely intentional that `line-height` cannot be set to less
		than 1.0 as opposed to it being a bug.

- Expected values in test assertions are hard-coded and currently rely on
the contents of the topspace.el file in some cases.
Since the previous commit altered topspace.el in that specific way, some of
the hard-coded values had to be updated for the tests to pass.

…de` (#22)

With linum-mode enabled, the line 1 indicator in the left margin would sometimes 
show too high up, above the actual line 1. 
The issue would occur when scrolling and topspace-height was zero before scrolling 
but greater than zero after scrolling. This prevents the issue.