Pivot Grid: gap in header borders

[imincheva]

Description

On one of the Pivot Grid demos there's a gap in the header border

NOTE: This is only reproduced when you dont have a column dimension

Steps to reproduce

1. Open Pivot Grid page (applicable for all themes and all sizes of the Pivot Grid)
2. Look at the second header row (first cell in the row says "Drop Row Fields here"

Result

The vertical borders aren't connected with the horizontal border underneath them

Expected result

The borders are supposed to be connected and no gap should appear. I believe the problem comes from the applied max-height on igx-grid-thead--virtualizationWrapper