Prevent PDF table rows from inheriting @rowsep from previous row #1486

In a CALS table, a rowsep signifies that a rule should be drawn *below* the
current row or entry. The XSLT code checks whether the preceding row or entry in
the table has a rowsep so that if there's a page break that immediately precedes
the current row, a border is drawn at the top of the page (that is, above the
current row/entry) to ensure that the rowsep value of the preceding row or entry
is respected. This seems to be relevant mostly with tables without <thead>.

Before this fix, the code looked for @rowsep values in *all* preceding rows.
Thus, if the row that immediately preceded the current row had a @rowsep value
of 1 and the row before that had a @rowsep value of 0, the code picked up both
of those values. Therefore, the value "10" was fed to the number() function, and
since 10 is a truthy value in XSLT, $needTopBorderOnBreak was set to 'true' and
as a result, every row below a row with @rowsep of 1 was drawn with a top
border.
Merge pull request #1552 from emth/rowsep

Prevent PDF table rows from inheriting @rowsep from previous row

Fixed #1486