lowercase header values to be consistent with FeedsCSVParser

Problem/Motivation
Feeds xls supports reading from CSV files, but I get different behaviour than the standard FeedsCSVParser.

Code examination revealed that the column names (from the header row) where not being handled in the same manner as FeedsCSVParser. FeedsCSVParser converts all header row values to lower case.

To reproduce, create a CSV file that has a mixture of uppercase and lowercase header row values.

Proposed resolution

Change the parseHeader function to apply a trim and a drupal__strtolower to each column header value in a manner similar to the corresponding routine in FeedsCSVParser.