renamed columns are not recognized with IFNULL

[vdun]

Example with IFNULL:

query error: no such column: c
Warning - There seems to be a "no such column" error, and -H (header line) exists. Please make sure that you are using the column names from the header line and not the default (cXX) column names. Another issue might be that the file contains a BOM. Files that are encoded with UTF8 and contain a BOM can be read by specifying `-e utf-9-sig` in the command line. Support for non-UTF8 encoding will be provided in the future.```