Tablesorter: colSpan property was incorrectly accessed

Due to mixing up of colspan attribute and colSpan property, the
condition always resolved to 1. This effectively made it a
headerIndex. Renamed it as such and moved the building of the
header/column mappings into the buildHeaders function, where it is
more suited.

Bug: 47654
Change-Id: I264afb565afbe555733bad499bad6f9c87894982
Merge "Tablesorter: colSpan property was incorrectly accessed"