Bugfix: CombineReports orders columns correctly

The previous test only covered the case where two columns
swapped indices.  The code was not correctly ordering the
columns when the column switch was more complex.  A more
complicated test case showed it was broken, and a small
change to the code fixed it again.
Merge pull request #64 from apregier/combine-reports-bugfix

Bugfix: CombineReports orders columns correctly