Fix scroll issue in Prototype version

[pfiller]

@kenearley @stof

As first reported (and fixed) by @jeslopalo in #1201. The Prototype and jQuery versions do not use the same logic and the jQuery version appears to have the correct version.

Currently, high_top is calculated as such.

//jquery
high_top = @result_highlight.position().top + @search_results.scrollTop()

//prototype
high_top = @result_highlight.positionedOffset().top

This change just makes the Prototype version incorporate scrollTop:

high_top = @result_highlight.positionedOffset().top + @search_results.scrollTop

[kenearley]

This looks fine to me, although I cannot recreate the bug.